Anybody, remember White Knight Chronicles? They had a system sorta like this. Signature moves were piecemeal and you had a certain amount of AP that could be allocated to each portion of your newly created move. Albeit, that wasn't really an MMO. It was online... and it kinda worked lol. I think all my moves were like STAB STAB STAB STAB. STAB STAB STAB lol.