I am looking a replacement for an RTS game I played for a long time... Battle for Middle Earth 2. I would like it to have a fantasy or medieval theme as well, like Age of Empires II back in the days.
I prefer the playstyle of the mentioned games and of WarCraft III, so I'd like to avoid games from the Total War series, wich seem to be too complicated and too big for my taste... I loved the style of StarCraft back then, too. I can't wait for SC2, but again, I'd prefer it to have a fantasy theme.
There are many fantasy games i'd like to recommend but unfortunately for you the fantasy genre is more dominant in the Turn based strategy games rather than in the RTS side... And thus most of the fantasy RTS games are older ones.
Judging from your preferences you could try
Age of Mythology.
Pretty much the same as Age of Empires with fantasy/mythical flavor added.
Dungeons and Dragons Dragonshard
Rise of Nations: Rise of Legends
More of a steampunk than fantasy game but good nonetheless.
It's a shortlist as you can see, I'm more into turn based / tactical strategy games myself