I just finished the game in normal and started a nightmare run and would like to say that I wish the perma-death aspect was at least separated from fast-monster/higher damage, maybe even as an 'iron man' toggle when you go to make a new save. The player movement speed while sprinting around feels pretty much perfect, and the faster monsters in nightmare felt much better and less trivial than in normal, and I also felt like my survivability exploded out of control in the second half of the game on normal so the double damage would probably be nice, but the perma-death tanks my motivation to actually play back through it.
Other than that, I think it'd be nice to be able to swap weapon mods once you've bought either of their new forms. You can't even have multiple saves running to try different weapons without starting over completely, and it's not like that would break the balance [certainly not as much as being able to run ILs for infinite xp and upgrade points]. It also seems like you should be able to replay the 2-3 level long chunks you do to progress in the campaign right out of the Workshop, and I can't really understand why you can't. The level select menu looks so much like it would do that that I was convinced it was either bugged or had to be unlocked by getting to a certain point in the game. As it is, you can't actually apply any weapon mod or spell purchases if you're just playing individual levels, you have to progress in the campaign to save those changes. Being able to replay level sets from the Workshop would probably fix that too.
Some quick things:
1 - Please add level stats like secrets on the hud or on the pause menu
This is definitely another thing I was looking for, convinced it was just somewhere I hadn't noticed for a long time. Really feel like the game would have a better difficulty curve and make actually exploring the levels more interesting if they saved which upgrade points you had already collected, so you can't get by just farming one you know is easy to get on a short level over and over, and actually had a reason to attempt to find secrets in the maps, but you can't really do that either if you don't know when you're done! And for a game with no quicksaves, putting it at the end loading screen doesn't help!