The only bad thing I could think of was that after killing the high priest it becomes very straight forward, after Moonmark tower level 2 you cant even go back any longer which kinda limits the experience in my opinion. Would be better to late in the game find things that can be used early on in the dungeon for example.
I got 10/14 secrets and as I mentioned earlier could have opened the gem door on level 16 (making it 11) but with the wrong gems (couldnt continue forward if I did).
I didnt like that the game ended as soon as you killed the last boss, since that mean don't get the xp for him if you want to import your chars later on. Would be better if you could chose to finish the game (stairs down open up again when you kill the boss or something and the game ends when you go down them).
I know you said the game wasn't tested on hard difficulty and I must say that showed. Maybe add a comment "Play on hard difficulty at your own risk". Very very hard at times but also rewarding of course when you finally beat the obstacle after many tries.
Overall I must say it's the best mod by far I've played. Yet again, awesome job! /bow
Edit: Also it took me exactly 10 hours to complete.