Let's design a basic RPG game engine

A Round is 6 seconds.

Each character has 5 attributes:

At character Creation, you start with 15 attribute points to distribute. You must have at least 1 point in each Attribute except Magic. The default maximum attribute is 6, but if you have the “Enhanced Attribute: X” Feat, you can increase your Attribute of that type above 6, but you must still spend 70 Attribute points to go from level 6 to level 7 in that attribute.

Strength
Controls Strength Save, Strength to deal damage, and damage resistance.
Brawling/Melee Combat
Lifting
Jumping
Push/Pull
etc

Vitality
Controls how much life you have and Vitality save. This number is multiplied by 10.
Resist Poison
Stamina
etc

Agility
Controls chance to avoid attacks and Agility Save.
Parry
Dodge
Climbing
Thrown Weapons
etc

Intelligence
Controls Intelligence Save and ability to read, write, and learn new bonus skill points on each level up. Intelligence below 2 makes a character illiterate.
Languages
Learning
etc

Magic
Controls Magic Save and magical damage and so forth. Controls how many Mana points you have. You get 10 Mana per attribute point in Magic.
See Below for School Descriptions

Attributes and skills grow with diminishing returns. On level ups (experience) you gain 10 Attribute points per character level plus one attribute point per Intelligence. You also gain 20 skill points plus one skill point per Intelligence. To gain in an attribute costs ten times as many attribute points as the attribute rank you are going TO, and to gain in a skill costs ten times as many skill points as the skill rank you are going to, except level zero which also costs ten skill points.

Okay, so this means if your strength is 3 and you want to go to 4 strength, it would cost you 40 attribute points.

If your Dodge skill is 4 and you want to go to 5, it will cost you 50 skill points.

To determine the difficulty of your rolls, subtract your attribute from 12 and that is the number you need to beat on your 2d6 die roll. Add your skill penalty or bonus to your die roll and add any difficulty modifiers to the difficulty.

If you are trying to shoot an enemy, then you roll your attack with the ranged weapon. Whatever you beat the difficulty by is added to the opponent’s difficulty to dodge. They roll their dodge and if they fail they get hit.

Melee combat uses the Parry skill, not the Dodge skill.

Unskilled rolls have a 1 point penalty.
Level zero rolls have neither a penalty nor a benefit.
Each level above zero has a 1 point benefit.

Magic,
When your Magic Attribute becomes 1 or greater, you gain one school of Magic at level zero with that character for free. Gaining a skill level in a magic school costs twice the usual number of skill points, and Gaining a new school costs 20 skill points.

Schools of Magic
There are many schools, and each school has at least 8 spells by one double-cost skill controlling that school.
Primary Elements:
Fire, Earth, Air, Water, Life, Death, Time, Nature, Trickery, Shapeshifting

Fire
Mirage
Cost: 2 mana
Casting Time: 3 seconds
Creates one plus one per three character levels illusions of yourself. When an enemy attacks you with a melee attack or ranged attack, there is a chance the attack strikes an illusion instead of yourself.
Duration: Three rounds plus one round per Magic Attribute.

Flame Dart
Cost: 2 mana
Casting Time: 1 second
Creates a flaming dart that strikes a single foe for damage equal to 1d6 per Magic Attribute.

Flaming Fury
Cost: 10 mana
Casting Time 3 seconds
Creates a flaming dart that strikes all foes in sight radius, dealing 1d6 per Magic Attribute to each struck foe.

Anti-Matter Ray
Cost: 10 mana
Casting Time: 6 seconds
Collects Anti-Matter and creates a beam of Gamma Rays to strike a single target, which penetrate Fire Resistance by 50%, Deals damage equal to 1d6 per Magic Attribute and multiplied by 1.5. Negative resistance becomes further bonus damage.

Heat Shield
Mana: 4
Casting Time 6 seconds
You gain a bonus to Strength to resist damage equal to half your Magic Attribute rounded down, and when an incoming attack deals damage to you, that attacker takes damage equal to your Magic Attribute.
Duration: 3 rounds plus one round per Magic Attribute.

Water

Absorb Mana
Mana Cost: 2 MP
Casting Time: 1 second
You steal an amount of mana from target enemy equal to 1d6 plus your Magic Attribute. If this increases your Mana above maximum, you lose one mana per round until your mana is no longer above maximum.

Douse
Mana Cost: 2MP
Casting Time: 1 second
Destroys all fire related buffs on target enemy.

Phase Shift
Mana: 4
Casting time: 6 seconds.
You transform into one of the phases of water for the duration. Gains appropriate bonuses.
Duration: 3 rounds plus 1 round per character level.

Ice bolt
Mana: 3
Casting Time: 2 seconds.
Deals 1d6 damage per Magic Attribute to target creature, and slows that creature by 33% for the next 3 rounds. That creature gains a 1 point penalty to all attacks for the duration.

Reflected Ray
Mana: 12
Casting Time: 6 seconds.
The same as Ice Bolt, except it hits all targets in sight radius.

Air
Flight
Mana: 5
Casting Time: 6 seconds.
Causes your entire party to fly, making them immune to Earth damage and move at twice normal speed. This stacks with Haste/Haste 2.

Lightning Bolt
Mana: 3
Casting Time: 1 second
Deals 1d6 damage to target creature per Magic Attribute. Also stuns the victim for one full round unless they make an Agility save.

Lightning Storm
Mana: 12
Casting Time: 6 seconds.
Deals 1d6 damage to all enemy creatures in sight radius per Magic Attribute. Also stuns all the victims for one round, unless they make an Agility save.

Teleport
Mana: 5
Casting Time: 1 second.
Teleport to a single location within line of sight or within 100 meters within your knowledge.

Trickery (Learns Enemy spells)
Town Portal
Mana: 5
Casting Time: 1 second
Creates a portal which teleports you to the last town you visited, Collapses when you return.

Reflect
Mana: 5
Casting Time: 6 seconds
Creates a barrier which reflects SOME enemy spells back at the caster.
Duration: 3 rounds plus one round per 3 character levels.

Etc.

Time
(Has no damage dealing spells, because they don’t need them.)

Slow
Mana: 2
Casting Time: 6 seconds.
For the next 3 rounds plus 1 round per 2 character levels, the victim loses half his attacks and suffers a 1 point penalty to all rolls, and 1 second is added to all casting times of Magic spells.

Slow 2
Mana: 10
Casting Time: 3 Seconds
the same as Slow, except it hits all enemies in sight radius.

Haste:
Mana: 5
Casting Time: 6 seconds
the friendly targets number of attacks per round are doubled and movement speed is doubled. That target also gains +1 to all rolls for the duration and spells take one less second to cast. This can’t reduce casting time below half a second.
Duration: 3 rounds plus 1 round per two caster levels.

Haste 2
Mana: 15
Casting Time: 3 seconds.
Same as Haste, except it hits all friendly targets…

Force Field
Mana: 5
Casting Time: 6 seconds
Traps a single target foe or Friend inside a Force Field for the duration. The trapped character cannot move, speak, or take any other actions for the duration, and cannot be attacked for the duration. However, if the character is regenerating they continue to regenerate for the duration.
Duration: 3 rounds plus one round per Magic Attribute.

Stop
Mana: 10
Casting Time: 6 seconds
Stops time for a single target for the Duration.
Duration: 3 rounds plus one round per Magic Attribute

Mass Stop
Mana: 15
Casting Time: 6 seconds
Stops time for all enemies in sight radius for the next 12 seconds.

Dispel
Mana: 5
Removes all buffs from target enemy creature or removes all debuffs from target friendly creature.

Life

Heal
Mana: 2
Casting Time: 3 seconds
Heals target friendly creature by 1d6 per magic Attribute.

Heal Wave
Mana: 10
Casting Time: 3 seconds
Heals all friendly allies in sight radius by 1d6 per magic Attribute.

Raise Dead:
Mana: 10
Casting Time: 6 seconds
Resurrects target dead mercenary for free with 50% Life.

Hold Undead
Mana: 8
Casting Time: 6 seconds
Stops time for all hostile undead in line of sight.
Duration: 3 rounds plus one round per Magic Attribute.

Death

Channel
Mana: 0
Casting time: 6 seconds
You lose 18 life, but gain 6 Mana.

Drain Life
Mana: 10
Casting Time: 6 second
Target foe loses life equal to 1d6 per Magic Attribute, and you gain that much life. This can’t increase your life above normal maximum.

Skeleton Warrior
Mana: 5
Casting Time: 6 seconds
Raises a skeleton to fight for you.
Duration: 24 hours or until slain.

Skeleton Mage
Mana: 8
Casting Time: 6 seconds
Raises a skeleton mage to fight for you. Has a random element from fire, ice, water, Earth.
Duration: 24 hours or until slain

Lesser Lich
Mana: 15
Casting Time: 9 seconds
Raises a Lesser Lich to fight for you. The Lich has a vorpal melee attack with a 10% chance to kill death-vulnerable foes in one hit. In addition, the Lich is immune to electricity and cold.
Duration: 24 hours or until slain.

Black Hole
Mana: 15
Casting Time: 9 seconds
Banishes all non-death-immune enemies to another dimension if they fail their Vitality save.

Feats
Every third character level you gain a feat. Each character also starts with one free feat and 25% resistance to one school of Magic.

Some feats include.

Enhanced Attribute
Allows Attributes above 6. Must be taken once for EACH attribute you wish to increase above 6.

Ambidextrous
You can use both hands equally well.

Magic Resistance
+25% May be taken only once. This applies to non-damage dealing spells.

Elemental Resistance
+25% This applies to elemental damage whether or not it is magical.

And so on.

I’m not done with this, but hopefully this gives the reader ideas.

This game is designed to use low stats, so stats don’t mushroom over time, and ALL rules of all abilities, attacks, saves, rolls, and weapons, etc, are displayed in-game, so the user doesn’t need a 3rd party site to understand the game.

Players who want to “Stat Max” should start with 6 or 7 (enhanced attribute) in Intelligence.

Character levels and XP
XP is earned with diminishing returns and character levels have diminishing returns.

Monster level difference // XP bonus or penalty

-7 or more // no XP gain
-5 or 6// 75% penalty
-3 to 4 // 50% penalty
-2 to 3 // 20% penalty
-1 to +1 no difference.
+2 to 6// + 20% XP
+7 to 10 // + 50% XP
+11 levels or more + 100%XP

Character levels
Level 0 Free
Level 1, 100xp
level 2, 300xp (200 more)
Level 3, 600xp
Level 4 1000xp
Level 5 1500xp
etc.

the first campaign would be capped at level 50.
the add-on expansion is capped at level 75.
Second add-on is capped at level 99.

This is intended to be “real time” but involve expert game mechanics on par with Advanced Dungeons and Dragons. You are intended to win through teamwork and good character builds, not potion spam.