Currently geo explosions look just like fusion explosions, with moho being a larger version. While that's kind of cool it doesn't really make sense, and the fact that geos and fusions explode the same way is boring.
So I came up with an idea; geos and mohos get smaller shock explosions, and instead shoot a fountain of kodachi-like fireballs of various sizes randomly over a larger area. That way geos would have a more volcano-like effect and could set random things on fire instead of just instakilling whatever happens to be immediatley adjacent. Mohos would just be a more spectacular version of that.
Note that damage should be high enough for it to be actually destructive rather than just annoying, and mohos should still seriously wreck stuff when killed.