Well, actual tibia progression Thais area was the first place to be created and it was "Tibia" and Fibula was made 2nd. I don't think either of those are amazing or a big step up over rook. Modern venore with coryms and stonerefiners is just where everyone power levels and would slightly kill the game.  However PoH and PoI would make a great quest area and my first thought on what to add.

Edron probably is a decent area to add to next though. Behemoth quest, Demon helmet quest, and Anni?! It avoids any of the more OP modern mobs (XP ratio), and keeps a more classic feel. Add in Orshabaal as a really rare spawn?! Makes the best gear Magic Sword (48), Stonecutter (50), Thunder hammer (49), +Mastermind Shield. Demon Helmet, Magic plate armor, Golden legs, Steel Boots (What would BoH actually do?). Those are all pretty iconic items and would make great content!

I like the 3 mob cap on spawns, but for anni, actually changing it to drop them into 6 demons at once?! Yes! Orshabaal being a rare spawn, he should be destroying everyone. They would both be long term goals and take a lot of work (if they are even possible? If you don't actually add vocations, allowing everyone to get ML 100+ and use ultimate healing, then its probably doable?) 

That is probably where you would have to end the game for the most part. Everything else you add after would be kind of pointless, except for POI and modern stuff.  I would still play everything and explore every area, but for most people who haven't played Tibia, if you suddenly added Carlin after they would just be like ??? what was the point of that?

One separate idea to add, I wanted some of the hidden levers to be secretly clickable! Clicking hidden lever to unlock next zone would have been pretty neat.  Might be a bit much because 99.999% of people who play this are not going to know every single lever and hidden tile in Tibia.