How about making a new breed of Fighting Golems. Golems that we could level up, customised with the spending points from leveling and maybe socketable to customise them further more with gems and such. Raise the stat cap to make them over powered if properly enhanced, socketed and trained.
To make the Golem one would need blacksmithy (socketing), tinkering (to make), alchemy (to add poison attacks, explosion attack and such), maybe find something that could be added with carpentry, like armor for exemple.
Special tools could be made available through quests for each profession, and be soul bound, same for the gem and component, that way, they could not be traded or sold until the golem was completely built.
Also make an arena where we could only use them at to fight and train, when they leave the arena they could auto shrink back to ensure they are not used to fight mobs or used in pvp.
We could make them have the choice of a few class like archer, melee and magic attacks, with either fire, cold, poison, energy and phys damage with a strong versus and weak versus system, that way a player would need more then just one. Kinda like a Pokemon tournament type but UO style!
I have a more detailed system/concept in mind i can give any gm's interested in working on the project.
 
Thx