Hey, thank you so much for writing such a massive feedback. It really means a lot to me, really. It's the first time I receive such a huge message with a lot of thoughtful and very valid criticism and it will tremendously help me to make a better experience.
I planned something else for the last boss: I wanted to make different layouts and arenas, something a bit like Dungeon Crawl Stone Soup  end floors (Old school roguelike, I really like the level design of it). Make it so when you arrive you have different variations with more pits and ennemies. Sadly, I got burned out (still had time left) and I was too scared to add another one, or to touch the fight, because the hour prior somebody softlocked themselves in the last level before the fight because I forgot to add pieces spawn points... This is something I really want to do in the future though !

The pieces are green or purple because they come from a different area. I wanted to make some kind of progression like this, thought it would be cool and mixing uup blood would be fun, I thought it was ugly the last day and wanted to removed it, and it's still there and I don't know why.
The targetting is broken, sometimes it does not show the correct information, sometimes the top left is wrong.... Again sorry for this. It's not possible to know which pieces can jump or not because of spaghetti code.

I do have to improve massively in all UI aspects. It's an area I struggle a lot into, and I really want to be better at it ! One of my goals this jam was to make something playable from start to finsh, and the user experience is still lacking to my taste.

The issue with incentivize the player to use as little pieces as possible is that it would make fights more tedious to me. You would have to spend more time to do more tedious strats, spending turn after turn and move a piec e at a time. This is something I want to avoid !
I want the player to make combos, do sacrifices if necessary to keep the turn as low as possible. I think it is funnier this way and why I added the par system. Unfortunately a lot of people didn't understand it (my fault of course definitely not theirs), and maybe it was not rewarding enough. Maybe I need to punish players for taking too long, but it would feel pretty arbitrary !

Game is not properly balanced after the first couple floors. I had very few feedback and test time for this and at the end I added extra pieces to the starting characters, which I feel makes a very high difference . The abilities might be overtuned as well. The last area may be too easy. But, maybe something about the formula is wrong, getting a piece each fight is too much and doesn't feel that good. These are things I would love to tackle on a bigger release than this. 

I do want to make it so sacrifice are necessary for later levels, at least 0.8 pieces per floor on average, if not more. This is a tricky spot to balance and the first time I have to do something like this ! I'm excited for what the future brings on that part.

Softlocks and lose conditions needs to be elegantly adressed, and I'm still trying to figure out a way.  Threefold rule is an idea but i feel like that would feel deeply unrewarding to not have a hard lose condition, not sure how to phrase that.

For progression I did not have anything planned like what you mention. What I thought was, after the shop, have 2 or 3 area choices, which would have very specific themes. The hole was an attempt at making this, an area with a lot of empty space ! Later on I want areas that only include some pieces, in order for the player to see as few different units in a game as possible (I'm trying to get that number under 15/20 but I'm not sure it would be feasible). 

I like shortcut :( Basically says that you have less fights in an area, you get less money in the end but get to keep your resources ? I think it's an interesting trade, maybe a bit strong. Passives needs to be reworked in general, I ran out of juice at the end, feeling sad for these as they do not feel great and inspired. I like the pin idea, it might be interesting to add in a shop as a side purchase! I like the system, would want to experiment with that. Maybe even remove the other passives!

Thank you so much for the lengthy message again, and sorry for the long response. Feel free to ignore that i'm juste rambling for myself !

Hey, nice question and great point ! 
I actually thought of this from the part of the definition "a departure from what is normal, usual, or expected, typically one that is unwelcome.".
I am rather not keen on doing stuff that are unwelcome to a player, though, and am not much into horror games and the like.
I spent one or two hour twisting the idea in my head and went "What if I could try to ""subvert"" the player's expectations by hijacking an existing game and trying to get something else out of it ? It's not a clever idea, it's not something that hasn't been done already it's like all the "What if x but instead it's y" stuff.
But I still wanted to have something that stayed true to the core game's principle. This is why at the base it's a chess game, and it's trying to stay as true as possible to the core loop "turn based strategy with planning"!
Also at the start it was more meant to be a Chess X hotline miami game but I'm not good at graphics so the only thing left from that idea is the blood splatters.