I want to try the fantasy version of TA that Cavedog made, even though many of the old reviews generally sounded disappointed with it.
Total Annihilation: Kingdoms? I played it. In fact I was the only person from my friends who finished all scenarios in single player campaign - and there was much more of them than in any other game. I've just found by googling there was 48 scenarios, pretty big number, and only a few starting ones were short and simple. I must say I liked it much more than original TA but that maybe was because of fact that I prefer fantasy settings over sci-fi settings. From what I remember, I enjoyed it but, unfortunately, I've never played it online.
I own a copy of TA: Kingdoms I can confirm that it is mediocre. However, they never skimp on their games, there's a ton of levels iirc.
I know this is solved, but you can mod the hell out of TA, making its replayability very high.
Can you, or someone else, recommend me good mods?
Honestly there's so much, from unit additions to full on game mechanics changes, you should really take a peek and see what you're interested in yourself.
I recall this being sort of the official hangout back in the day: http://www.tauniverse.com
but this one comes up on Google too (seems to have a good writeup of the kinds of stuff out there too): http://www.rakrent.com/rtsc/html/ta4.htm
I'd check those two sites, do some googling to make sure they're the real deal and don't shovel viruses and try some stuff out. See if you can get a friend to play with you, that's what made this game fun.