I would focus on three things:
1) Addition of servers with more diverse forms to play Tibia: Another "Retro" PvP, but with older PvP settings, a "slower" server (one where there is not any kind of experience boost, neither any option to buy it on stores, except for the ones obtained on world events), a "fast" server (with permanent double experience, just like Aurera and Aurea used to be, and all boosts are liberated) and a non-restricted world (one where weapons and runes got no level restriction, just like the old times used to be). I would maintain the regular servers that we have today, however, and allow people that like to buy xp boosts and etc be able to use it.
2) Reformulation on how new additions shall be implemented and a very meticulous revision on newer areas and sprites added on the past few year: Some new content for me is just poorly conceived/childish (like the new grooves inserted on Mainland. They look awful with the natural vegetation and were a very bad insertion to the lore... I'm ok with stones and trees that talk, but they seem to me very unfitting for the Tibian lore) and not to mention some new monsters that basically utilizes old sprites with "modded" stats and names (mino cults do not match with the Mintwallin design, I would review the whole dungeon and AT LEAST change the color of the powerful mino cults mobs... It feels so poor the way that they designed, copying regular sprites...).
3) Complete renewal of the Free Account settings and addition of new fun and/or interesting spells for all vocations, and at the same time, a change on how spells are adjusted to every vocation (for example, why knights can't have more spells regarding their shielding capabilities since early on? They could lose their cure poison spell however and have the need to use cure poison runes... We could talk about things regarding it all day long). Situational and fun spells like the one that had Sorcs enchanting Staffs were cool to see at least, and had a very situational use. This specific change would be vital, since we need to attract new players and call back the old ones to this game, so I would try to apply any kind of attempt to add more stuff to all vocation (like a second promotion or anything else)