-License D&D rules, use the rules, don't worry about ensuring the game fits the D&D world.
-Don't break the bank on graphics; a good art style with an isometric view and you're gravy.
No & No.
The whole D&D RPG model is plain risible and tired. People end up obsessing about the mechanistic aspects of stats, levels, magic items and min maxing and narrative engagement becomes secondary. About the best D&D based RPG (Planescape) succeeds because it largely subverts the formula by making the unnamed one unkillable, and therefore the narrative had more weight to it.
As regards isometric, please no. That whole removed tactical aspect in DA:O was dire. Way too many encounters in that game were built around this notion of success through Dark soulsesque trial and error, especially with some of the later big group fights to the extent that you needed to basically lose a few times in order to figure out
how to game the system in terms of which AoE spells to cast where and when in order to overcome the placed opposition. Works OK in a game like Dark souls where from a narrative perspective you're effectively in a purgatory, but not so well when you're supposed to be a mortal in a living breathing world with just the one life to lose. Victory through replay is a terrible approach to encounter design. A player should always possess the means to overcome a situation as presented, and if they do fail it should be as a result of their own tactical actions, not due to a lack of options or foresight.
What games as a medium do increasingly well is atmosphere and immersion, and that's achieved best by keeping you as the player in the game space and less in the interface. I want my Role Playing Games, to be
Role
Playing
Games, not poor replicators of P&P RPG mechanics.