Not only would a D&D game sell, and not only would a turn based game sell, but combining the two would bring even MORE profits.
Turn-based games are a freaking goldmine on the DS and PSP. Dungeons and Dragons has huge following amongst RPG gamers.
And based on what do you come to this conclusion?
The less than stellar reviews and sales of some D&D games (TOEE, D&D Tactics, Dragonshard, DDO), showing that D&D is not a "must buy" brand, or the small number of turn based games coming out and only on handhelds while there was practically no turn based RPG title (without real time combat) on any major plattform for years (Maybe a Front Mission, but see next paragraph)?
Or is it that all the tactical turn based rpgs on handhelds having a lot simpler combat mechanics than 4E has with its eventual 10+ powers per character with different recharge timers, trigger, hit&miss effects, pushing, shifting, teleporting, marks, etc?
What would work is a tactical RPG, similar to Bahamut Lagoon, Front Mission, etc. But as that is just a series of combats where you can equip your characters between battles I don't consider them real RPGs. But even that won't happen, as they are (nearly?) exclusively made by asian studios which haven't shown any interest in D&D.
It comes down to having a western IP best (but not overly much) suited for a more JRPG like game. Chance of that happening is very low.
Unless of course the combat system is changed radically, more than the 3E system was to make it VG mainstream compatible.
And then there is of course the question who would make such a game. Atari is out after the lawsuit as in Bandai and EA has its own IP. That were the big RPG companies. Bethesda's RPGs are not very similar to D&D and also has its own IP. Square/Enix has its Final Fantasy and probably no real interest in an western IP. Whats left from the big ones are Activision/Blizzard and Ubisoft which both don't produce RPGs apart from WoW. And I doubt any smaller publisher or company without publisher can pay for the D&D license. They rather take a cheap russion novel IP and cobble together a system for it (happens quite often).
The best thing you likely can hope for is a action game with D&D flavour like Dark Alliance.
But when you are soo conviced that D&D is a huge brand in the VG industry, then the news of the Lawsuit must have created huge waves. Would you kindly link me to any of those huge discussion on a VG website debating about it?