I think it'd be an interesting idea!
what if the champ has connections to the very life essence of Runeterra and carries some of that essence with them as the form of a companion, where their neutral form is a ball of energy that provides a certain set of abilities/strengths depending on what ability is cast- ie: each ability transforms it into a different astral beast that gives buffs/abilities.
there could be a wolf one that could have a similar effect as WW with his bloodlust for a short amount of time and give a very slight speed boost to a hidden enemy, or an enemy of low hp nearby for 5 seconds or so, and provide a small lethality/magic pen buff on the first damaging attack to the tracked enemy.
there could be a stag that provides a trample ability, that briefly knocks up the targeted enemy and slows for .75 seconds after the knockup, which then bleeds them on your first damaging attack to the enemy champion.
there could also be a bear that, when activated, gives a percentage health shield for a brief duration, and gives an attack speed boost for 5 seconds.
for the ult... I have no idea, perhaps it could merely be a steroid for their other abilities, and maybe change how long each effect lasts.