2 champs in 1?
As everything ever, it's probably been thought of and mentioned but, I really think it would be amazing if there was a new champion that was actually a duo tag team.
one of them could be an Ap tank/support/mage, and the other an Ad fighter/assassin/marksman.
they would obviously have a tag team move that swapped the characters into primary and secondary controls with a bonus effect and mechanic.
whichever character is not under summoners control could act as a guardian unit like lulu's pix, semi-uncontrollable like malzahar voidlings, or could be a controllable pet like shacos clone.
there are many ways to synergize teh characters, like if one dies the other could take over and possibly with a long cooldown revive their teamate, or they must wait for that teamate to respawn(yes respawn AI/cpu), something to that effect. it could be as simple as if one dies, they both die, or they could share the same health pool.
I know this is a tough concept to implement but dang, how ridiculously fun would it be?