I have 4 ideias for a mod, 2 simples and 2 more complex.
1 and 2 are the simple ones:

1 - its strange to me that when going the alt path, when you turn ghost form you can't acess boss challenge rooms, it should be possible, and maybe this happens to other itens that transform you into lost too? not sure about the second part

2 - True randomizer: a randomizer that can even decide your destination, i know some sites do it but implementing it in game, to make the random be able to choose btw normal and tainted chars and even give/forcing you to go on determined directions like challenges do, they make so you could only go alt path, or give you both key pieces etc.

3 - Leash Them: how about an item like a leash, that lets you kinda control were your familiars look and move? the leash could put them in front of you at a certain distance and make them shot in that direction, the leash could also stop/limite movement of familiars that bounce on the room or go on straight lines, aka bob's brain, gurdy etc making then go in that direction but only until the leash permit, and maybe this could cause the familiar to pull isaac a bit in that direction like some dogs do to they owners

4- Make Eyes work more correctly / make stapler better?: okay this one is VERY hard, i think, because we would need to create a WHOLE new stat, for me it always felt odd how you shoot from both eyes but its not very clear and some interactions/synergies feel strange, the best example of this is the item Stapler and the charecther cain, both make so you can only shoot from the left eye, but if you get Blood cloth for example. that buffes the left eye, it only makes half of the shots buffed when in theory it should be all shots, after all, all shots are from the left eye, and there are plenty of other examples that could be said, like polyphemos + stapler, so maybe there is a way to make this work as inteded in a fun way? creating unique synergies XD