Take a look through here.

There was a decent Headmaster upgrade kit for Universe Hardhead a while back. It'll be pretty hard-to-find/pricey now though.

There are a few different heads for the Classics Bumblebee mold, to turn them into Bumper, Hubcap, or Goldbug. I have the Beelzeboss "Growing Pains" Goldbug upgrade for United Bumblebee and and it's pretty good.
There's also a few for the latest Thrilling 30 Bumblebee mold, but I don't know all that much about them.

If you include the Voyager Bludgeon as a CHUG then there's a great upgrade kit with a new head and swords from Headrobots. There's also supposed to be one coming out soon from Perfect Effect.

There's an upgrade kit for Voyager Blitzwing - new head, new gun, and a fix for the shoulders.

There are quite a few extra weapon sets for Classics Voyager Optimus Prime. And of course a couple of trailers.

Going way back there's the FansProject City Commander and Protector armors, and the Crossfire upgrade set for the Energon Bruticus mold. They'll be pretty pricey, though with MP Magnus and a new 3P Bruticus coming out the price might drop. FansProject also did an upgrade kit for Classics Cliffjumper, but that'll be even harder/pricier to get.