well, HP fire is an interesting idea (gen 5 for Ferothorn, right?) but the main problem is not only do fire and water get redundant coverage, but omastar is best used in rain. (i think either fighting or rock are cool, if you think that grass and electric are overdone).
also, im not sure, but i would probably ask the simple questions thread to point you in the right direction. If i had to ask someone, i would probably ask Diabolico.