So I've been thinking a lot about it as far as spells go there one of three options:
1 keep them the way they are, you start with one spell, de leveling to level 13 (6 levels in) gives you ult etc
2 start with all of your spells and slowly loose them and see how the game play goes with all champs fully built level 1 with only one spell
or
3 level up your spells like normal but they degrade in some respect as they level up (start in their stongest form and end game they are in their weakest form)
(personally i would think option one would be the best, you still get more spells over time and make them stronger, like your build, but the strength of the champion is gradually decreasing, plus it would be less work for Riot)
As for the de leveling ive thought of two options either
1 exp de levels you so you have to choose between fast gold and fast de leveling, or next to no gold but keeping your levels high longer, however after a certain amount of time out of combat you start to de level more rapidly (that way you dont get people waiting in base at level 18 while everyone else fights and gets weaker)
2 you de level solely in time spent out of combat, so exp is lost during time spent dead, and if you are spending a lot of time walking from one lane to another. Fighting minions or champions could either freeze the clock on you de leveling or reverse it slightly (though not so significantly that champs could just stay in lane phase all game trying to not de level