Not that anyone asked, but if I were going to redesign that game with minimal effort on the developers' part:
- Being invincible after being hit would also make Beck invincible to instant-death hazards and instant-death attacks. Bottomless pits are still death, obviously.
- Reduce the knockback from being hit to about 80% of what it is now.
- Disabled enemies would count as intangible. Touching them would collect them, and all shots would go through them. If you didn't collect them fast enough they would come back alive, just like they do now. That time could even be made shorter.
- Dashing into an enemy that has very little health remaining (1.5-3 lemons' worth, would need testing) would kill them and collect them. Dashing into them otherwise would hurt Beck. This status would be indicated by outlining them in red or making them flash between normal and disabled looks, or something.
- He would be limited to 2 airdashes per jump. (I'd make it 1, but there are sections where at least 2 are required.) Being hit would give you back both, grabbing a ring would give you back both. [removed because of mcpeanuts' valid point that this is an important part of what fun MN9 does have]
- Shorten minimum dash/airdash distance to 1 frame, if you actually release the button that quickly.
- Remove mantling (ledge grab) in favor of the below option. Grabbing rings would still need to be present because levels are designed around that.
- With at least 1 airdash remaining, when next to a wall Beck can walljump upward by pressing Jump, which will use up an airdash. (This could just use his flip animation from ledge grab->Up, and is in lieu of implementing full Megaman-style wall cling and walljump while still preventing you dying.)
- If a boss hit you with what is now an instant-kill attack and you have more than 1HP remaining, it would bring you down to 1HP rather than killing you. (This is closer to the intended spirit than just making those attacks do 50% damage, and probably more fun because those could still one-shot you with 50% health left. Might be worth reducing the damage AND having them not kill you from above 1HP.)
- Shots would travel the full length of the screen, but stop counting toward the bullet limit/allow you to fire another shot after they reached the range where they now disappear.
- Shots that went farther than a little bit offscreen would IMMEDIATELY DISAPPEAR, or at least immediately not count toward the bullet limit.
- Extra energy tanks would use themselves as soon as Beck received fatal damage, like Auto mode Reserve Tanks from Super Metroid, rather than requiring the player to manually use them. They go away when you die, this is the fairest way for them to work with that restriction.
- Switching weapons would be possible at any time, no matter what Beck is doing! I would not even require the addition of weapon switching from the pause menu, since that would be a new UI screen. (Optional - change controls to allow switching by holding the weapon-switch button and pressing left/right. Holding the button would pause the world. Hint, look at MMZA's weapon switching. :^)
- Remove the crouch-dash command and just let him automatically enter crouch-dash state when he reaches a short opening.
- Allow pressing the Dash button while on a ladder to disengage from the ladder and dash without requiring jumping first.
- Allow pressing the Shoot or Jump buttons to cut off the current line of dialogue in a cutscene and advance past it. (Just fake like it finished, in code.) Press-start-to-skip-the-whole-thing is not the only useful feature for cutscenes.
- Mute the in-level voiceover for dialogue that occurs while players are playing, even if the rest of the dialogue is spoken.
Optional - redo some of the boss powers (sword, bulldozer) so they don't just grant you invincibility frames forever if you continue using them / mashing.
That's just the basics, but I think that's enough to vastly improve how it feels, at least to me.
None of this requires level, enemy, or boss redesigns. If this were the Z-Engine, all that is doable in well under a week, so I imagine one of their more-experienced designers and a programmer could do it in about that time.