Personally, I prefer maps like Ice and Flame.

Campaigns are interesting, but in my humble opinion, they should be implemented better through small or medium-sized maps. Large or very large maps should be reserved for the last and second-to-last missions. Overloading them with quests, in my opinion, is also not a good idea – it's more suitable for separate scenarios. 3-10 quests per map are more than enough, in my opinion.
But there's a problem with carrying over progress... in all the amateur campaigns I've seen, the campaign itself is implemented through a series of maps, in each of which the player starts from scratch. Is there a way to bypass this? A partial solution is to start with a Sphinx or arenas (libraries). As mentioned above, spells could be a possible solution – perhaps a rune that, when stepped on, erases certain spells...
I would also like to see some of the Heroes 3 campaigns reworked for Heroes 5, for example, campaigns like Eternal Love.

Among the factions, I find Demons interesting. I like the Knights. Necromancers are also enjoyable, but they are difficult to balance depending on what the player will raise and the transformations of creatures.
I don't like Mages – the battles are tedious. I also don't particularly like Dwarves.
The Tower is neutral.
Lizards and Elves – it depends on the map... sometimes I like them, sometimes I don't.