Alas, it is true that continuities outside of GEEWUN (and to a lesser extent the IDW comics and BEast Wars) don't really have the same appreciation. Obviously show-accuracy isn't a problem as the show models were basically carbon copies of the toys, I'm just thinking of ways to make a decent toy into a great one. I think there's a lot of people who have the Energon Constructicon Maximus and like it but wish it was a tad better (IIRC it was the best recieved out of the 3 Energon combiners) so I don't think the market is totally non-existant, just smaller than the market for GEEWUN figures (although I reckon if people started making non-G1-based products and they were good it could very well increase the demand for more variety in the 3P market, thus allowing for a great deal more innovation and representation of other continuities and even totally original ideas- Planet X's Genesis opened the door for WFC/FOC stuff, and now we have Planet X's Dinobots, 2 different FOC Bruticus upgrade kits and that upcoming line of WFC/FOC Legends class figures).
TL;DR: Yes, at present the market for non-G1 3P Transformers is small, but that could very well change and it would be cool if it did.
(In any case the chances of CM getting an upgrade kit are very slim, but it would be nice if he did).
Anyway, here's how I'd do it:
For the 2 extra limb-bots, I'd make one of them a rockwheel trencher and the other a front-pouring cement truck.
In addition the kit would come with 2 feet (which would peg onto the sides of Steamhammer's legs/excavator mode via his knee minicon points, and would also store a bunch of little 5mm tools and gizmos in addition to hiding the hands a la Quantron), shoulder blocks to give him more poseability in combined mode (attaching to the feet unit's ankle joints and disguising as missile pods/searchlights whilst decombined), various extra weapons and equipment that could be pegged onto the individual vehicle modes and also combined into various super-weapons (or components, connectors etc) for combined mode, and a flail on a chain (that could become a wrecking ball for the crane bot when not combined).
In combined mode, the trencher-bot would become the arm and the mixer-bot the leg. In arm mode, teh trencher-bot could retract/remove his hand and fold up the rockwheel for SAW HAND ACTION (because replacing parts of ones body with cutting instruments is cool). In addition to the various weapons formed out of the equipment of the smaller 'bots, the mixer-bot's barrel could detach and transform into a huge cannon for him.
So...yeah. I dunno, I think he'd look pretty neat, and more combiners is always fun. Steel Core was awesome and he was totally original, so...