I don't know how I would build him, but I am wondering if that -2 to strength is really that bad considering there is a possibility of a tank who can cast mirror image or even turn invisible if things get too hairy.

some combination of either barbarian and fighter or paladin and fighter could really prove effective, I think. What do you think?

Yes to the first, no to the second.

A deep gnome rocks as a tanks, simply due to that wonderful +4 generic AC bonus. The addition of mirror image and blur are great.

However, I must say that they make great tanks as monks. Consider this: potential 20 DEX and 20 WIS, on top of a +4 generic AC. With maxed stats, you start with 25 AC. With items and accepting xp penalties in exchange for mage and cleric levels, this guy can reach the coveted 72 AC mark.

Why is a 72 AC "coveted"?

Yes, a dg makes a good tank. Monk, monk, monk.

A 72 AC is coveted for those maniacs who spend a lot of time on HoF. A 72 AC is essentially unhittable on HoF because the BAB bonus that monsters get is limited to about 50. There are a few exceptions to the capped BAB bonus - the Guardian comes to mind.