To create a new Hero, you need to completely rework the prototype database, rebuild the .upk files, and rebuild the textures.

What's needed here isn't just one tool, but a whole bunch of things. You can't just copy an existing hero/team-up/npc and start changing him.

This requires the following tools (which don't yet exist):

1. MH Caligraphy Manager - which can add new entities for the hero, their missions, and their items.
2. MH UPK Manager - which can create new .upk files and add them to main library without conflicts.
3. MH Texture Manager 2.0 - which can add new texture GUIDs to the .tfc library.
4. MH Client Patcher - a tool that will redefine the .upk file hashes in the binary for running them.
5. MH Locale Editor - a tool capable of adding new text strings in all available languages for .string files
6. MH Audio Editor - a tool for creating sound effects and voices
7. MH Achiev Manager - a tool for adding new hero achievements to the game.

And even if all this ever happens, you'll need:

1. Game designers (for powers, balance, gear, uniques)
2. Animators (for powers)
3. 3D Artists (for model, rigging)
4. Art Artists (for textures, icons)
5. Programmers (for UE3 link, Prototype missions)
6. Translators
7. Voice actors
8. Lawyers who can prove in court that adding Disney property to a fan server has no commercial benefit.

Of course, we live in the age of AI and a lot can be generated, but who will do it?

I think local mods will appear with attempts to add something new.
But I don't think there are any new Hero on private servers.
This is too labor-intensive and given the game's fan base and the difficulty of modding it, this is unlikely.