Too many
I would love sequels to Psychonauts, Republic Commando, Banjo-Tooie, Beyond Good & Evil, Zack & Wiki, and Baldur's Gate: Dark Alliance II. In fact, now that I think about it, Microsoft could easily fund some of these sequels, especially considering Spencer's recent comments regarding exclusive software. If Microsoft greenlit Psychonauts 2, Republic Commando 2, and Banjo-Threeie, plus Jade Empire 2 and Kameo 2 (a game Microsoft previously cancelled), it would go a long way toward diversifying its portfolio and bringing in new (and old) fans.
Other series due for a renaissance include TimeSplitters, Turok, and Wipeout. It would be really cool if Nintendo would purchase the rights to Turok from Disney Interactive (?) and assign the IP to Retro. It would be a homecoming of sorts, and fill a major gap in Nintendo's software line-up.