I think the end of the days of infinite mana could pave the way for a new road to something better than how things were before. Champions should not all be the same, or close to it. But how can we make these champions that now waste like 4k of their hard earned gold just getting enough mana regeneration to not run out of mana really fast? Well, what if there were an alternate version of something like Archangel's Staff, except it gave you a bigger bonus the more MP5 you have? Not to mention we need a new AP item after DFG was removed. Consider something like this:
Fairy charm + fairy charm + needlessly large rod + 1040 gold (or something like that) =
+50% base mana regeneration
Unique passive: Grants +.25% base mana regeneration for each spell cast/mana expenditure. Max: +100% base mana regeneration.
Unique passive: Grants AP equal to twice your MP5.
Maybe some unique active at maximum charge but if so, it's probably going to need a lower AP gain from the above passive or a higher gold cost to make this thing.
The idea is that the really mana hungry champions should be able to get more of a reward for having to build all of that extra MP5. This way, they won't be nearly as bad endgame since they'll at least still be able to deal more damage the more MP5 they built, so it's much less wasted. They drawback? This item is pretty weak when you first get it and you are not going to have a very strong midgame if you get this item rather early. And late, it's going to be a pain to charge it up.