Hey, thank you for playing, and such detailed feedback!
>Hopefully I can find some new skills for him.
You can unlock second class skills for every class by doing quests tagged as (class), for that.
>I have a feeling firestarter is OP?
Firestarter have some power, and his bottle skill seems a bit OP sometimes, but every class has different strengths and weaknesses. I will see for more balance later in development, also I think that you are playing on normal difficulty, hard one will make it more dangerous. And there will be a harsh moment at the 5th floor, where your current char (according to your screenshot) won't survive for sure, but you will see a skeleton near the entrance to that dangerous room as a warning.
>pickups could be more exciting
Do you mean more often, or better quality loot? On hard you get better quality loot more often.
>the later quests could have clearer objectives
-Mysteries Of Mystic Cultists-
You need to kill blue cultists, they have a chance to drop a key, or a boss, he has 100% chance drop, then you need to go near big room with skeletons on level 3-1 (room before the room with 5 cultists) and use your key there from inventory. Will think how to make it more clear but not too much.
-The Mystery Of Fireballs-
This is the quest to teach your firestarter his second skill, you need to collect cultist notes from red cultists, then study them (look at quest item description), that will make a page of report, you will need 4 pages (it mentioned in page quest item description) to make a full report. Will think how to make it more clear.
>fullscreen and borderless
Window mode works really strange and incorrectly, so I skipped that part, but you can switch between fullscreen and bordeless by pressing Alt+Enter for now.
>Do you mean more often, or better quality loot? On hard you get better quality loot more often.
I mean maybe have more variety in the stats on the loot you find. from what I've played, loot progression seems pretty linear. if you had lets say an amor that gave more magic power or more attack speed in the same tier, it could give more room for building the character you want. maybe you do have more variety like this but I haven't seen it.
>-Mysteries Of Mystic Cultists-
maybe just tell the player in which area the quest needs to be completed? I can see someone spending a lot of time in an area thinking this is where the quest happens when in fact it's on another floor. just an idea, I understand how cryptic quests can be fun.