Future Darkin Concept

Balancing nightmare here but what if..

A new champion had the ability to allow one dead teammate to revive them self as a Darkin or some sort of avatar of a creature from another realm.

• The Revived team mate would have a new character portrait, all new abilities and so forth.
• You could either limit the time (making it fairly short) but give him the ability to dash across the map and get into battles very fast or allow the newborn Darkin (or whatever) to play as that champion for the rest of the game.
• I imagine adaptive stats would be used for ability ratios. Item based stats could be included similar to the way Riot has done in the past with Jhin and Ashe.
• Implications of pro play might be cheering for your favorite pro or at least the one your favorite team trust with this very powerful character.
• The summoning ritual could be feel like a team "quest" to complete to allow them to unlock this power.
• Whether if its a one and done thing, repeatable, or permanent would determine the amount of power allowed in the kit.
• Power could be built around synergy between the original champion and the transformed teammate. OR the opposite, there's room to weave in some lore where there is dis-synergy. Say if the two players got to close to each other the original character consumes the avatar and ends the state. So if you wanted to maintain it you would opt into split push strategies.

Just a random thoughts I had that I felt were worth throwing out into the board cosmos.