inspiration key rune concept: Opposite day
Concept: Pick a champion, all of their kit abilities convert to adaptive AP or AD at adaptive efficiency. So you'd convert the stats first directly through the adaptive calculation (1.2 ad for ever 2 ap) scaling, convert the damage type, and be done. Certain champs might need specific tweaks later on but that could be numerically hotfixed if, for example, something like Tristana or Diana got too strong.
This would allow for interesting things like AD nidalee, AP garen, and a lot of more interesting builds but at the expense of a keystone. It would be a choice you'd pick before the match and would unlock new builds and higher tiers of mastery on a champion.
Baseline most champions have an 'expected' default position, so maybe reversing scaling on those who have inverse scaling would be a deminished return. Example again would be tristana E. But again, maybe it wouldn't be so bad since like... you know... she'd be losing a keystone for it.
I can't imagine any reason why this would be too strong, only reasons it would be a bit weaker in general than most picks, but I can see it as valuable after getting counter picked.