If I were billionaire rich, after using most of it on useful things, I'd be happy to pay too much to have them make a great version of Tun Faire from Glen Cook's Garrett P.I. series. (Although I'd wish I could have a few edits in books 7-9, and maybe skip 10-on except for Pular Singe).
Relatively low magic by DnD standards (maybe like an e8 game) in an urban fantasy setting with tons of different races butting heads. And at least some psionics. To do classic crime noir right would you need decent rules for knocking people out when surprised?
If I couldn't get that, I'd vote Wuxia.