Well what I would enjoy is a simple OT with realistic ambitions. All I want is fun. If you want something concrete I suggest you work on your npcs, the cities needs citizens obviously (though it's weird that there are so many people living in cities, after all tibia is a fantasy universe which is taking place kinda during a medieval epoch, and during the Middle Ages in around the 500-1500s close to everyone lived in the country). Npcs with rich dialogues. As for the exp rate, original tibia is way too slow, 10x should be fine.
Custom sprites and spell system? Not necessary, but absolutely adding some awesome spells would be cool.
Tibia client version? Mounts can be cool, but the way you can walk through other people in PZ. UGGH I hate that, aswell as the anti-trap system, including the disgusting player swaping system T.T. It's horrible. 8.6 OTs are by far the most popular to play, though the reason is that most boters can only bot on 8.6 and lower versions. But i'd prefer 8.6 anyway, don't mind people boting that much, maybe higher version if it can be without the sucky things I mentioned.