FrogReaver
The most respectful and polite poster ever
Remove feats and multiclassing. They're the lifeblood of powergaming in 5e, and optional systems besides.
Caution: won't fix glassbowls
Meh. Just shifts all powergamers into playing rogues, paladins and casters. Personally as a DM I'd rather deal with an optimized high damage party of barbarians and archer fighters as it's easier to throw more enemies at them than a party of mostly full casters.
It's much easier to engineer a challenge for high damage dealing characters than a party of full spell casters IMO.