D4 as an isometric ARPG is still an RPG, and as such should honor some of the core elements of a role-playing game.
Here are some tips blizzard for making a historically good RPG :
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Allow players to make bad decisions. i.e. Let them go into dangerous areas when they aren’t properly prepared. Don’t be afraid to let players fail.
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Story elements should be optional, and available if pursued. As a blood thirsty barbarian, i may not want to help the protagonist and would rather go smash skulls. Let me role play instead or being forced into watching your subpar animated cartoon.
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Let character skill growth be non-linear, and multifaceted, to help a character establish a unique identity.
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Let character builds have a wide range of strengths and weaknesses. For example, DPS, durability, healing utility, dps endurance, healing endurance, crowd control, mobility utility, treasure+ utility, party-dependance, party-independence, dungeon traversing utility, scouting utility, elemental resistances, light generation, etc… (further discussed here)
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Gear’s contribution to combat efficacy should vary by build and class. The contribution of gear to a characters skill identity should be smaller than that provided by experience and skill growth.
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A good RPG allows the characters backstory to be open-ended, as well as his/her reasons for fighting. i.e. let the player decide if the necromancer is a reformed lunatic imbued with dark energy or greed motivated power hungry practitioner of dark magic.
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Role playing emotes should be available. As a barbarian i should be able to choose when and if i want to spit on something instead of the auto-spit feature found in D3.
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Story choices should be a thing. If its a role playing game, various story paths should be an option, per the “role-play”.
TLDR; Good RPGs Give players an abundance of meaningful and consequential choices.