*spoilerama*
what if when you beat normal mode, you unlocked:
a) a traditional hardmode where enemies have a bit more hp and new tougher patterns
b) whatever else you're planning, idk but knowing this game it's gonna be fantastic
but when you beat hardmode, in addition to just difficulties you get a customization menu with options like randomization (for stuff like guns, items, enemies, enemy attacks...) and attack pattern speed
that way beating normal mode gives a logical hardmode that fits its name more, but beating that hardmode gives even more replayability to ANY difficulty instead of "oh wait guess I beat everything now"