Zelda: Phantom Hourglass and I guess Pokemon have a lot of depth. Obviously Pokemon isn't aimed at the older audience but it does supply 20+ hours of game play and if you want to collect all of the monsters probably closer to 100 hours.
If you want guns and can't find Metroid I don't know. I'd wait until COD 4 and Contra 4 come out. (They are both coming out this month) There's also a Goldeneye game that I found semi-enjoyable. I'm not sure if you'll be able to find it though. It's pretty old.