I think including a hard mode with quiksave and other helpers disabled would be a win-win. I mean a mode that can't be changed mid-game.
Of course disabling the quiksave would mean there needs to be a good checkpoint system, or an equivalent of it. Abe's Exoddus lacked it, unfortunately.
I also think that AE's pseudo health system (Sligs don't hit abe on their first shot, but rather on the third one) works better than NnT's, mainly because it's more believable and less obvious. I mean, how can you survive a few shots to the chest being basically naked? It doesn't make much sense
You can also tweak (reasonably) the number of
licks it takes for fleeches to reach the center of Abe, for different difficulties.
My last thought is it generally is a bad practice (and a common one, unfortunately) to design the game around the easiest/easy difficulty and *then* design the harder modes. It usually leaves both sides (More and less experienced gamers) unhappy, with the veterans being the most unhappy, as the hard modes become very... cheap. Like, let's exaggerate a little, mines blinking green for only a nanosecond. The original Oddworld games were definitely challenging, but they
never felt cheap at the time.
It's not an easy job finding a fair balance, I wish you all the best!