Cross shouldn't be able to fire with other mods trinkets, because there is no effect, and there are a huge number of trinkets in Fiend Folio alone

There is still the effect of being able to loot that trinket on kill on occasions but as far as the effects go, I think the proper way to do would be using random effects instead when none are found and/or properly handling data from other mods. Mods may define their data for my mechanics but it hasn't been done yet. I may just do it myself for the most common ones in the future.

I think that it would kinda make sense if tiny trinkets were less effective. Because they are smol

That was the initial design intent tbf, "trinkets with 0.5 trinket multiplier value" but sadly the API is super lacking on that part so I couldn't do it. Since then, I've changed them a bit because of the shift of design and had them less common as a result, resulting in what is playable atm.

I really loved the effect of preventing devil deals chances to reduce in Ira Item. Could you make one item, even it's only avaiable via console, that's prevents reduce devil and angel deals when take red damage? I play coop with my youth brother and he talks every time about a mod that could do this.. I would appreciate to play more chill runs with him c:

If you want I could show you how to do it on your own on the modding of isaac discord (feel free to mp me)

you forgot the double outlines on quite a few sprites (and it looks really weird with things with some double outlines and some with single outlines)

while some like deconstructer are completely missing outlines in certain places

Originally posted by Hooded:

while some like deconstructer are completely missing outlines in certain places

There are quite a few placeholder sprites on the mod still, those will get updated along the way
The rotten doesn't have costumes yet, for example

Soul of Cross glow looks weird. You know, soul itself is brown and glow is kinda red

All the runes are basically recolored versions of existing runes (visually). I expect them to be redone at some point!

It will be great if you add unlocks

Sadly I can't say when I'll work on an achievement system. That requires a lot of art and spriting has always been a rough ride with Beyond. I'm not an artist myself and it's been hard to find spriters.
I'm currently focused on mod compatibility with other big mods and will resume working on researched items after that. I might switch to achievements if artists suddenly appear but nothing is less certain.

Maybe add a possibility drop new trinkets if it possible. Because if you have many trinkets in a room you just accidentally change a trinket and cant return it back.

Do you mean for mini trinkets? It's intended not to be able to change them at will! In particular it's a huge constraint to handle when playing The Little (Tainted Cross) and that's part of their design.