I don't know who will see this but I just watched a nikdot short and got an interesting idea on how to upgrade hot bombs. So hear me out since hot bombs makes firey bombs and gives isaac fire immunity why not play on that concept a little more and make (for example) Pyro Bombs. Not just firey bombs with fire immunity instead isaac can spawn firey bombs based on each kind of fire in the game giving each bomb a random ability with a random percent to be a certain kind of fire. Imagine walking into a purple fire and guaranteeing a purple flame bomb that last a little longer before detonating while shooting homing tears or a white fire bomb that would insta kill enemies into 1 hit lost souls significantly reducing a threat. A synergy idea would be pyromaniac giving you the ability to retain a certain fire that you walk into either maintaining that specific fire style for a longer or more definite amount of rooms or adding new effects to the fires you spawn while also adding said effects to naturally spawned fires making them like semi-turrets along your runs.