Spirit champion
Just an idea, needs refining...
This champion at spawn (and respawn) would be a spirit, unable to interact with the game world: no skills, no attacks, not killable, can just move and explore. (Maybe also semi transparent or even fully invisible to enemies?)
In order to interact with the physical world he has to possess a monster or a minion. Possessing different monsters gives him the ability to attack and cast spells (with stats and effects depending on what he possessed) but also makes it possible to kill him (by killing enough of his hosts - just one or does he just lose some of his own hp when the body is damaged?).
He can only leave the body he possessed if that body is killed. He also can't recall while possessing something. He can though sacrifice some of his own hp to get rid of his body.
Killing his body gives the killer the gold he'd get normally by killing that creature.
He gets credited the kill of a monster or creep he possessed only if he manages to walk it all the way to his fountain (making "stealing dragon" quite literal for him XD) and only after 6.
(His early jungling would probably consist of taking the red buff monster to the blue buff golem and duking it out XD)
The first thing I'd think about this is "wouldn't possessing the dragon at 2.30 make him op?" Well, maybe. But A) The dragon is slow. B) He can't heal while possessing and can't be killed by his own team. C) Towers can (obviously) attack him. D) He can't just walk home and call it a day (he needs level 6 for that).
All of this together makes me think that he may not be op early when possessing the first dragon: either he stands around doing nothing (which is essentially a 4v5 for the enemy) or it will eventually result in a guarenteed free dragon for the enemy.
(I do wonder if a dragon host with no sustain would survive enough full jungle clears to be brought in the fountain at 6).