You already did what I would do, turn back.
Other than that, and not knowing the aircraft myself.
Full rich - you did that
Carburettor heater
Fuel pump
Check fuel - switch to fullest (Piper 28)
Try magnetos, switch between right and left to see if that makes any difference.
But like I always say to my own students, grab the damn checklists if you're not about to die right away... I mean, if you have time for it.
Since I just looked up the checklist, I can see I forgot one item (depending on going for "roughness" or "power loss"):
Check the primer is locked....
That's why we have checklists.