Try not to fall in the pit of stat inflation either; Overly tanky enemies are unfun to deal with (Tedium does not equal fun difficulty) and making everything a one-hit death can be frustrating and takes the reward out of health capsules.
Upgraded bosses would be an interesting change... Of course changing the attack patterns would be interesting, or speeding them up or something else... But an easier path would be to attempt to alter boss areas, or in some cases bring out more than one boss? Like imagine if you had to deal with Balrog AND Misery at once. Or if you gave MALCO a boss fight by reskinning something?
Altering an arena rather than the boss itself can already bring a lot of changes yeah.
Spots of water to slow you down, rougher terrain, offscreen enemies tossing projectiles, the little rolling guys...