You can hire heroes from the Campaigns on your own maps or maps created by others! You can download Heroes 5 maps with heroes from the Campaigns. I played on one of those!

On the Nival forum, there is a mod for version 2.0 that allows you to hire heroes from the Campaigns on any map. I just don't know if it works in other versions. And I don't really want to download it. These heroes are just too cool. Markal and Isabel are super-supportive heroes, and Argail, Railag, and Faydaen are excellent main heroes. True, against Faydaen, a player with an available slot can fill it with an unupgraded level 1 unit. All the archers will shoot at him. I also don't know how good the mage is.

Added 10 minutes later

Faydaen may be the best, but what will happen if he doesn't have archers or druids in his army?

In the Campaign, he always has them. And in a regular game, you can buy a hero of another race with archers at the beginning of the game and give them to Faydaen. It would be great if there was a mage with a bunch of gremlins. Dugald with archers will also do.