1. It would be an interesting option but less versatile than you think. Shooting enemies is literally the most boring part of JJ2 single player and it's good that they die fast. What makes a level interesting is platforming, exploring, finding secrets, trying not to get hit, etc. Not shooting a turtle 50 times to kill it.

2. That would be a good thing, but again it's pretty limited. It's not easy to use bosses in a way that's creative and challenging because there are only so many of them, and their patterns are all pretty basic. And modifying things beyond simple stuff is extremely difficult and no one is ever going to bother putting in the effort.

3. Use 1.24 if this bothers you. Everyone has it.

4. I like how lori is an utter failure. I want to see some levels designed specifically for that. I've decided if I ever make another SP pack, the story is going to have multiple characters, and in some (possibly all) levels you'll be forced to play as one of them. A level designed specifically around a character's limitations (and advantages) would be interesting, I think. I've already experimented with this a bit by making a spaz-only level, and I like how it opens up a lot of designs that are impossible otherwise.

5. This isn't really single player specific though. One thing that would be neat is to have the clear water that beta JJ2 had. Lighting worked with that, and it also looked a bit better in my opinion.