They've got 3 types of leadership already - normal, magic, undead. Add 2 more and turn them into Elemental leadership.
If you want to go full Pokemon theme then everybody should have access to every Pokemon but each square has different ones and some are available only through rare sites, but the elements all take different leadership so they'll be much more/less effective under certain trainers.
As for magic, you should have 'levels' of trainers/pokemon and as they get higher level they cost much more but have much more magic. Like a level 1 trainer should only have 1 level of 1 magic, a level 50 trainer should have L2/L3 in multiple paths.
Higher level pokemon can only be summoned by trainers entering sites requiring higher level magic of that path.