Just keep the normal goombas,koopas,boos,snifits,piranha plants,shy guys,hammer bros,chain chomps,thwomps,whomps,bob-ombs etc. but upgrade them a bit.Like you could give Koopa Troopas this armoured shell with rocket launchers with heat seeking missiles....but Mario & Luigi will find a way to thwart them...Goombas with armor on that repels jump attacks,Boos could make up illusions to trick Mario & get him off the trail of finding Peach,Chain Chomps could get bigger,Snifits shoot grenades instead of tiny cannonballs,Thwomps & Whomps need to get faster at squashing Mario,Bob-mobs gotta like pump themselves up like bigger than Bowser then explode....but Mario & Luigi will get through all them.Why can't I be the bad guy in Super Mario instead?
Do you know when we die,we''ll be boos too?