LEVELING
Finite levels with endgame becoming available before max level like Diablo II and PoE. Infinite levels are just endless grind and as various communities have shown are not really welcomed. In blizzards case look at AP grind. This applies only to power leveling, if this grind is for skins, mounts, pets or anything non-power related it’s more acceptable.
SKILLS & TALENTS
More complexity on both of these. More talents and some of them should be impactful – these should be located at or near the end of a talent branch. Also no easy re-alocation of points. Choices should have some meaning unlike Diablo III. You could offer a reset at every major patch/expansion/update.
Skills could have some choices every five or more lvls. This choice should be permanent and not easily changed. Items could be used for this and each could allow you to change only one of these choices. It should be tradeable
ITEMIZATION
Ancient items concept as a droppable items is not so good. Many have stated that before. Instead they could be augmented legendaries. Each season you introduce 2-3 per class and each can have more than one upgrade. Also it should be upgraded through a lore-friendly quest that tells us the story of said legendary. (I don’t know how much work is needed per item but it would be cool). This gives a character to each and every ancient legendary, it provides a personalization option for the character and it is not invalidates legendaries. Also it provides a way for you to focus on some items/builds each season as you said during the blizzcon. As a bonus fun(?) you could make the quest to upgrade for one option per item hidden and players must search for it. After the upgrade the ancient can not be re-crafted. For a different upgrade you must find a new one.
Rares should be viable. Not every legendary should outshine every rare for all stats.
Also add more complexity to items and stas. Simple generic attack and defense stat is not good. it’s boring.
More runewords / more complexity or start with jewels/gems and add runewords later when you have seen how we react and what builds we create. That way you can balance them better and introduce them smoother.
SOURCES OF POWER
First bring back stats like Diablo II or a similar system. Automatic leveling is sometimes good but clearly this community wants not to be bored when leveling. We want some of the complexity, power and mistakes that were made in D II. And not changeable skills and talents – at least not easily, as mentioned before either through some items or at the start of each major patch/expansion/ or something (but not league).
As more multiple power sources you could have different “leveling“ systems. Lets say you have normal levels and a bloodline level where you can awakend your nephelem, demonic or angelic blood with you – blood is the key as you have said :). Or you even can make mythic items like the artifact weapons in legion (but not able to choose all the updates – similar power tree).
KEYED DUNGEONS
If this strategizing is changing build then no. You should strategize by changing a few items. Using potions that increase resistances or something like that. Not changing character. Look at pen & paper rpgs about the preparation that players go through when entering dungeons. They buy stuff, equip different items and prepare different spells but they don’t throw their diviner because they want an evoker, or Forge cleric for a Light one (Domains) (D&D5e).
Also we see so many monsters in the world but never how they came to be there in the first place. There could be some portals around the world though which they come. These portals should not remain for a long time (15-25 mins) and should not be visible in the map before they are found. With their appearance the general area around them could have increased monster density and higher levels, that should be the only clue that player have about them, it is up to them to found it before the timer expires. But when they are found it becomes visible in the map, a world boss appears and if it defeated then the portal remains open for all players to enter. Of course this leads to some layer in hell. This area should be difficult, huge and chaotic. Also for further confusion you do not enter through a standard point but are randomly teleported inside the map (parties should go together). And for further fun this area is PVP area, with semi-world bosses, elites and extremely good loot. And you can play with the layer, it could be huge chaotic or maze-like, layered, different rooms or arenas, timed or even having constant damage done to you wherever you are. You can explore your sadistic side.
TLDR: Ancients should be upgraded legendaries accessible to all through a quest with multiple powers but you choose only one of them, more complexity, finite levels.