For MTW2- As mentioned before, Stainless Steel is worth going after. Larger map than in Vanilla, as well as several new factions (Aragonese, Kievan Rus, Templars and more) and is a great mod to play. It's the only MTW2 mod I've ever played but I enjoyed it thoroughly. If you want the link to download it it's here. FYI MTW2 needs to be patched to 1.2 for it, as the page says:

http://www.twcenter.net/forums/showthread.php?t=110425

As for a BI mod, the only one I've ever played is IBFD. It's a fair mod, and while a lot of work's been put into it the legionaries' stats outdo just about any barbarian units, eventually making it possible to completely steamroll literally thousands of barbarians with a few hundred (4000 killed by 1200, 100-200 casualties as romans, 3500-3900 barbarians wiped out) All the same it's worth giving a go since I've never tried playing as the barbarians and it might be funner that way.

Enjoy