I'm a bit surprised by your opinions on the Mutant Mudds DLC. I mean, I loved the hell out of that game in its entirety and was a bit let down by the fact that Grannie was unlocked after beating it, as it is a huge difficulty cushion. But those new levels took everything that should make that character a difficulty cushion and amped it up. Now, none of the levels were as hard as any of Max's levels, but they still had a number of their own, innovative ideas and I thought it was a nice way to end the run of the game. Sixty levels isn't bad at all.
However, we already know of the boss battles making an appearance in Mutant Mudds 2, and I personally hoe that we see three new, different powerups in the game, in addition to some new enemy types. I think that enemies, obstacles, and powerups form the trifecta of difficulty in the first game, especially how V-land and C-lands focus on giving you only one powerup that you need to exploit and throwing more difficult obstacles and enemies in your way to compensate for it. I just hope that same formula is used to create the new games.