Investigator can be done as a Rogue archetype for mine. No need for a whole new class.
A 'pet' class could be awesome. Akin to the summoner/ eidelon vibe in PF.
Obviously a martial controller. Something less fight-y than the PDK, but more controller-y and leader-y. Just the one iteration of extra attack, but more warlord/ marshal type class features. Auras, buffs, boosts and that kind of thing.
A magic item focussed PC. Alchemist/ artificer/ runesmith type. Buffs the party with potions and temporary magic items. Can make (DM dependant) permanent ones.
A Sorcerer archtype that doesnt instantly turn you into a scaled monster, or randomly have you becoming a pot plant. Think 'Arcana' bloodline. Something that a Mystaran could use..
Psionics. [Please capture some old school feel in there (I wanna see Ego whip, Id insinuation etc)]
A redone ranger. The class offers nothing past 5th level thats worth sticking around for.
A kensai/ weapon master archeype for the Monk. Variant martial arts that grant slightly different bonuses and abilities to 1st level Monks (reaction trip attack instead of bonus action attack, pin damage on a grapple instead of bonus damage attack etc)
A Wizard archetype that isnt a school specialist. Universalist type. Ignores magic resistance, and maybe gets a capstone at 14th of concentrating on two spells at once.
Off the top of my head.