Also think, Formulas.
if you come to think about it, most battle systems, if you take away all the eye candy, and the animations, and whatnot, in the end
you're left with cold hard equations and formulas.
while I haven't done any battle systems myself, I can still say parameters (such as HP, ATK, DEF, etc...) play a big role on this, before even
considering choosing what type of battle system you'd like to do, ask yourself this:
-how will I handle damage? how does a player's ATK and DEF affect the standard formula?
-how much can a player control on this system? can he move? is he stationary? is it on turns?
-how is a battle dissected? do enemies attack first? what is my priority at the start of the turn?
....the list goes on,
but if I were to choose an absolute starting point I'd say, WIN/LOSE conditions, when does a player win a battle, and when does he lose?
assuming that you're choosing the standard win/lose condition of having all members in one side have 0 HP, next would be, deciding how
to deal damage, selecting a standard formula, sticking to it and whatnot, then add conditions to it,
basically you gotta start from a very simplistic addition/subtraction system, then once you have stabilized it work your way up to the
details, from simple to complex, you get to decide as you build it.