Sports tracker is a must have! Even though there is already a similar app for Maemo 4, called eCoach, Sports tracker is much more mature, and it already has a large community.
As well as some kind of Office suite, not necessarily M$. But in light of recent Nokia M$ alignment, and if the rumors are true that Nokia will ditch Symbian in favor of Maemo, it'll be interesting to see how will M$ react.
N-Gage would also be nice boost for the device.