If you are looking for nations to mod in MA, i'd say Man (though Xeiter did an MA Man balance mod awhile back) and Machaka need the love.
Man can often make a go of it in the early and mid game, but they are notorious for collapsing like a stuck balloon in the later game. So much so that their late game weakness is partially responsible for their early game success- people leave them for later 'cause they know they are easy prey.
I'm not really sure why Machaka never wins, they seem solid to me, but i have not played them since my very first net game of DomII and only fought them once in Dom3. Machaka's also pretty "vanilla." They have no national spells at all and lots of their stuff could be just be updated to the levels of more recent nations. Seems to me you could do all sorts of fun jungle stuff involving lions and creepy-crawlies of all description. I'm too lazy to mod, but i've thought about doing Machaka alot and they'd be my pick.
The other nations that have not won in MA are Bandar (everyone knows this is because of their horrible PD
), Atlantis (can't hold their own underwater vs the other two water nations), Vanheim and Eriu. Haven't put much thought into it, but i suspect that the last two may suffer from being very capitol dependent in an age when that is not the norm. The two are quite similar and Vanheim used be considered a top nation.
As to strong nations, well, Pythium's won 7 of 43 game in the HoH, that's almost 1/6 and speaks for itself. People say angels, but i point out that Marignon has angels as well so it's more than that.
Pangaea also has a noticeable win total- 5.5 on the HoH. Personally, i think Ermor and Marignon are under appreciated powerhouses and Caelum is my least favorite opponent in any age.