One of the hardest games ever made? Most definitely. One of the most satisfying beat-em ups? Absotively posilutely. I don't know why this series never made it past 3 game sin the classic era. I'm sure now, if somebody were to come along and try to remake it, they'd mess it all up with today's obsession with "dark, gritty badassery"; the toads would all be grey, they'd be packing Uzi's, and every character would be voiced by either Jason Statham, or someone who sounds remarkably like Jason Statham.
In my humble opinion, a modern-day sequel to Battletoads would play a bit like the Arkham Asylum games, at least as far as the combat controls are concerned. Let the toads fight their way through hordes of enemies and ride vehicles across a bunch of colourful locations in both single or co-op mode (without the nasty friendly fire that made the original so difficult to master). Also, make them eat flies more often. That never gets old.