My personal graveyard 
OG Xbox:
JET SET RADIO FUTURE, Phantom Crash, Mechassault 1 and 2, Fatal Frame 1 and 2 (OG Xbox versions are definitive with exclusive endings and other content), Amped 1 and 2 (licensing issues, I know…), Rallisport Challenge 2, obscure JP region stuff you would need a JP OG Xbox (or softmodded console) for (Magatama, Tenerezza, SMT Nine)
360
SHMUPS (not my image)
(some of these actually are in, and City Connection has been porting shmups recently, with Deathsmiles 1+2 coming next month, so hopefully these can get preserved too)
Dynasty Warriors series (was hoping for Gundam Warriors), Armored Core series, Lollipop Chainsaw (we got most other Grasshopper games. This one is a licensing issue for sure though), El Shaddai, Eternal Sonata (this one seemed like a fairly big request)