Just a strange idea here but... What about adding Robobomber into the giant stage without changing the block layout to Robobomber's stage's layout (which is made so that he won't step on any softblocks). Maybe even make it so that his giant stage version has a mounted laser turret under him that shoots every 3/4 a second every so often at softblocks and players under him with a 3 shot volley. The shot shouldn't stun players and would remove 2 powerups per hit.

I also feel as though the mounts need a little spicing up.My favourite mounts are the ones from advance mode (or whatever the most complex mode is called) in Party Bomberman or maybe just strap on the tweaks that Super Bomberman 5 gave to the Luis?

EDIT: Might be a good idea to replace Robobomber's balls with piston legs if you can do rotating sprites. What I mean by that is 2 sticks that slide in and out of each other when the leg stretches out.