The heroes introduced in the Heroes & Villains DLC can be modded, changing them or adding new ones.
You can download a simple example mod here, for you to study and modify. You can also look at the already existing heroes in data/crossplay/heroes/HeroType. You can probably figure things out just from that.
The rest of this post attempts to document everything exhaustively, so don't feel that you have to have read all of it to start modding heroes! Reading "Basic Hero Structure" and "Recruit Hooks" and then whatever you think you need should be enough to start.
You can find detailed documentation on how modding works here, but here's the short version: A mod is a folder containing an info.json file, a logo.png file, and a bunch of folders containing JSON files with the same structure as the data folder in the game. You will also need a strings/en.properties file for the text in your mod, and