Boy, Kingdom Hearts needs Spinoff HD Collection bad. Bring in Birth By Sleep, Dream Drop Distance, and only the cutscenes from Re:Coded and 358/2 Days. Star Wars Battlefront HD Collection needs to happen too, especially since old original Xbox Live went down. Someone suggested Phantasy Star Online, which is also a great idea.
It's pretty out there, but I would like to see .hack and/or .hack//GU HD Collections. All four original games (Infection through Quarantine) or the GU trilogy on single disks. And I know I'd like this and it's somewhat possible because on G.U. Rebirth's bonus disk they actually have HD remakes of some cutscenes from the original games (well, PS2-era HD).
I would also REALLY like to see the Golden Sun trilogy get an HD Collection and bring it to consoles. It looks beautiful even with the GCN's GBA player, I can't even imagine....
What else...? Soul Calibur 2 HD Remake as standalone with online multiplayer and create-a-character would be worth more than SC 3, 4, and 5 put together. Star Wars KOTOR HD, since it doesn't really emulate that well on 360. A Disgaea HD Collection would give an infinite amount of playable hours. Chrono Trigger+Cross HD Collection. Tales of HD Collection would probably take up 8 disks lol.
But if we're going off the deep end, how about a Legend of Zelda HD Collection? Yes, it seems like they rerelease Zelda every other Tuesday, but screw it. I would indeed pay money to see OOT's 3DS remake along with Majora's Mask HD and Wind Waker HD on one disk. The three best Zelda games brought to full, glorious HD. I would indeed pay for that.
I will say this, though. A lot of great must-have games already have much-deserved HD Collections and/or remakes, like Shadow of the Colossus, some of MGS, Beyond Good and Evil and good lord, Hydro Thunder Hurricane (well, part sequel part remake).