That's what I'd call an excellent idea. I used to play in Dwi (D'Hara), FEI (underdog life in Cathay and later OW) and Rio in BT. I liked them all for different reasons, but I'm totally open to try different stuff in different continents and realms.
On BT, from what I hear (am not in the realm myself) Rio has changed a lot, so that may not be the best choice as it will ruin your good memmory of the realm prob. Nothoi is currently getting their asses kicked by the daimons and actually has a few active chars, so you could join there. For daimon worshipping you could go to Spearhold (no idea about the activity level there). Thalmarkin will be interesting in the future I think, but once realms are destroyed you can always move up there.
On EC, there are actually a few options from what I gather. Based on what I've seen on the chat, Perdan seems to be the realm with the highest activity and conversation level (just typing it already hurts) and it has enough (potential) enemies left to ensure enough wars. Vix as a democracy should be active also, but I'm hearing mixed stories about the realm being fun and boring. So I'm not sure about that realm. In the far south you have Xavax as biggest realm, but so far the most fun and action seems to be expected in Perdan/Vix/Oligarch based on current behavior. I play the ruler of Oligarch myself and activity levels there are decent, with a few very active chars and a few decently active ones and it has more than enough enemies to ensure war for a very long time. The idea is to create a more fluent diplomacy there, without the long lasting alliances. But I might of course be slightly biased towards my own realm.
On Dwilight, I would advise not going to Swordfell or Arnor tbh. From what I've seen they're not the best of ralms. I hear that D'hara has some RP'ers left that are still active, which could make it fun. Luria Nova is in the process of reclaiming lands, but some of its most active chars have left. Still there are enough opportunities there and some active chars left (Aldrakar just returned which I always enjoy greatly). Westgard is fighting constantly against the monsters, but I've got no idea how the realm is internally. But at least your char will see a lot of action.
