In a recent interview with The House of the Dead: Overkill producer Neil McEwen, he admitted that the team at developer Headstrong Games would be interested in doing a Wii sequel to another Sega franchise, Virtua Cop. With Virtua Cop 3 having last been seen in arcades back in 2003, ít’d be a good time for Virtua Cop 4 to return to consoles as another Wii-exclusive rail shooter.
We've spoke to Neil ourselves and he mentioned this to us aswell, Overkill has room for sequels and I would hope that SEGA won't miss that trick. Headstrong have created a whole new lease of life for the franchise and I'd be very surprised if we didn't see another HOTD game from Headstrong.
However, there's always Virtua Cop if they wanted to do something slightly different - that series doesn't really have as much scope for a total reboot, but it is another series that would fit nicely on Wii.
We'd expect an announcement sometime before summer on what Headstrong are doing next for Wii, until then this is pure speculation.