Auto-translated
In my opinion, the ideal game should combine all the best features from previous installments and their expansions (WOG), plus new elements.
Personally, I'm waiting for the perfect game based on 3.59, with everything it offers, but using the engine from either 4 or 5.
Level 3, unit upgrades! Definitely, and preferably with a skill tree.
A skill tree in the castle (more well-designed than in 4).
Caravans, without a doubt, and composite artifacts with the ability to rearrange them according to the player's choice. Initial selection of magic spheres, like in Age of Wonders, and...
most importantly, the implementation of all this should cater to the needs of gamers (although perfect balance is ideally unattainable).