Double Dragon and Ninja Gaiden. 
It seriously would be cool to have those games on Turbo/PCE with arcade faithful and upgraded versions. I like the upgraded graphics of Double Dragon Genesis, but they did a terrible job with the music.
Indeed.  Both DD & NG could easily be done.  As it is, NG never got a console port, the closest thing was the Lynx version, along with the VC version of it which I did purchase.
Someone mentioned STUN Runner(which, techinicaly is an arcade game).  Originally, I just didn't think the Turbo could do it, since we can't do true scaling(I'm assuming the Lynx uses sprites & tiles for its version), & we really can't push any poly's(it was a polygonal game, wasn't it, or was it something different).  However, Gunboat & Falcom have "some" poly's.  Maybe it could be done with a mix of tiles & poly's?
Chip's Challenge would be cool, & AFAIK it's not on any other system.  Toki would rock hardcore.  I kind of think of it as a Lynx game, since the Lynx was the system that exposed me to it, but I know there's an arcade game, as well as Genesis & NES.  Just in general, Toki would be awesome!
Oh, & speaking of Zaku, I did hear that someone involved with that game, is now working on a PCE project, but I don't know any more then that.....perhaps a port or sequal of Zaku?  Should be easy!
And APB would be great, though, it's also not a Lynx exclusive.  Xenophobe however is kind of it's own version of the original game, so that'd be cool.