Some remaining thoughts:
1) If the amount of loot to be grabbed exceeds your backpack space, maybe convert any items left behind into 1/4th their sell value, so you at least get something for your trouble. So if it normally would sell for 12 gold, the conversion turns it into 3 gold.
2) Deeper floor level should dramatically increase loot drop quality, to encourage more risk v reward dungeon delves.
3) The boss fight should be the same difficulty regardless of dungeon depth and should only be available once per run. Otherwise, its way too easy to farm them on floor level 1, with a level 5 group, and then get a ton of easily acquired powerful items.
4) For rerolling the passive, perhaps change the cost based on average character level. So if your 4 heroes are an average of level 3, maybe reduce the cost by 25%. Thus at level 4+ average, it costs 1000 gold, but it costs 250 gold less, per average level below 4 (so 250 at level 1), but also increase it per level above 4. So average level of 6, would cost 1500 gold.
5) Make it so that the rerolled passive is stronger than what you could find normally at character creation. Thus you might want to attempt to reroll it more frequently. Perhaps the passive is 10% stronger per level above level 1? So if you rerolled it for a level 7 hero, the passive would be 70% stronger than normal.
Otherwise, shall wait for next release.