Adding RPG elements is from a design perspective 1000X easier than modifying classes and achieving the level of balance that Era / Retail have. Perfect example of this is look no further than SoD; what a wreck. Its basically Retail mechanics in the Vanilla world but its tuned very very poorly. This has a lot to do with the team size, the lack of design background, etc. Getting a game in the same quality level in respect to class mechanical balance as Era or Retail (presently) is not an easy task.
Overall the best path forward for what you’re kinda looking for is MoP game client somewhere in the 5.2 - 5.3 patch class balance state with “Vanilla” like quests, and in game requirements.
If I were to build an EZ mode RPG that’s exactly what I would do, because MoP is 10X easier from a design perspective to tune and balance because it does not have traditional talent trees, it has more mechanics but they’re fairly straight forward, yet are easy to play and very very hard to master in a similar way to “Vanilla”, additionally the rotations are somewhat unique for PVE and there are lots of setups, counters and counterplay possibilities in PVP.
Mechanically the raiding was really fun, the classes offered a lot of opportunity regardless of class or chosen spec, and the same could also be said for Arena / BG comps in that patch range.
The only thing you could really improve on are the RPG elements in the game world by using more scatterbrain vanilla like quests, but leverage MoP era technology because it never had Shards.. tho it does have phasing.. phasing is lame, so dump that off a cliff to its death. You could so interesting things like require players actually travel to the dungeons, make resources in game matter, IE mana, rage, energy, combo points, chi, holy power. Additionally make consumables more a thing in PVE similar to early OG Vanilla where they’re needed to do the content rather than BiS your parse.
Overall, Era is a great game, but to build off it using retail like mechanics just ends up in SoD, and the existing classic team really lacks the Class / mechanics design chops needed for that level of game. MoP is the way forward for them, tho I suspect they’re not likely to do well with that either because again they suffer an unfortunate case of Retail thinking; its unfortunate because I think they’re really nice, spirited and passionate people.. Designers for classes however they’re not.
HOWEVER, with that criticism, they are really really really damn good at making PVE stuff, this is where they should focus, and this is why if you used BASE 5.2 or 5.3 MoP, and built content fully for that Core class mechanics set / game rules set.. You would easily have a gold mine on your hands, because you could then offer excellent PVE, Rated BG’s and Areana and casual BG’s that work and need zero balance passes.
This assumes the Devs DO NOT MESS WITH ITEMIZATION LIMITS AND BUILD SUPER MEGA POWER GEAR.. like they have done in SoD that has effectively broken the game.
Restraint is important in that respect, gear is a blessing and a curse, too weak and things are painful.. IE Greys / most greens in Vanilla but too powerful IE Naxx+ loot, GG well played also.