The priest group probably has the easiest "fix", give the druids a "channel nature" (which would be wild shape for circle of the moon druids, summon a pet for shepherd druids, etc.) and you are 90% done, since the three classes now have a unifying feature (channel).
I hope the unifying feature for the warrior class is some kind of bloodied feature that lets you become more superhuman for long fights. Anything that applies to rangers or paladins (like fighting styles or making more than one attack a turn) would be insufficient for a unifying feature for the warrior group, and the bloodied idea isn't the only one out there, but most are actually pretty big, and I am guessing they will go easy (like expertise and channel) first.
I don't know what the unifying feature for mages will be, but, for the love of Lolth, please don't make it yet another spell slot recharging system. That is just creative bankruptcy. I had rather they stole the alchemist from the artificer and say wizards make potions by following recipes, sorcerers do it by instinct, and warlocks get their potions by DoorDash from their patrons.... Again, something like that (unless they go with yet another spell slot recovery system) would be big, so I think they will go easy first.