Not sure if this had been suggested or not, but what if the champion's ult has some danger, some risk attached to ulting. Probably on a toggle that uses health as a resource.
Say, when the champ goes supernova, each second or so uses an increasing amount of health. The longer the ability is toggled on, the more power is gained and the more health is used, eventually leading to a self-inflicted death if not toggled off. This would create a high risk, high reward situation.
Perhaps there would be a mechanic that would allow the champ to stay in the fight longer in the empowered form, like from ability or auto attacks against enemies that would augment the health costs or regen health. This mechanic would demand good understanding of how much self-punishment the champ could take and could be countered by good cc management and kiting.