I've been wanting to do somehting with hippowdon myself, the RR one. His body is a nice touch for any deck that lets a LVL X sit out (shaymin, floatzel, etc). But I'm mostly drawn to his first attack. Its a lot like the old blisseys attack, where you keep pulling back energies and hitting for them. This idea could ironically work with blissey from PL, to keep dropping a fighting energy, heal by 20, and pull it back with Save Sand.
This could also be a decent attack when using DCE. If you send up a fresh hippowdon, no energies on it, and attach a DCE for the turn, you can hit for 50 (70 with belt), and get back a fighting. now he has 3 energies on him. next turn, attach a fighting and swing with ground quake (as long as you dont have too many basics).
I think the X is just a tech, for when you're going against something with tons of energies on the field, or tons of tools. Or both. Could really mess SP up bad, if theyre really relying on those Energy Gains. Make them waste their cyrus to get them back, and not get poketurn or whatnot instead.
I have the same problem, though. I'm really not sure WHAT i wanna run it with.