Add opacity modifier tool

Add a tool or brush mode that exclusively changes the opacity of images.

Not just decrease the opacity like the erase tool, but also be able to increase opacity as well.

Use a mask, right? You can go from 0% to 100% opacity with a mask and can brush as you like.

Not the whole image, specific areas or pixels, like how the brushes work

You can paint brush or any tool on a layer mask (or mask layer I don’t know the terminology) or even paste an image to act as an opacity pattern. Black being full transparency and white full opacity. You can even use filters on your mask/alpha layer this way (blur, contrast, curves …).

If you have specific use cases not covered by the use aof layer mask I’d be curious to read about these.

Typically I used layer mask ib gimp to achieve this result :
but it’s just one use case

The thing I’d be curious about is if you use a wacom or such input device, can a sigle button can be binded to switch between a layer and it’s mask layer.

This is something I would use layer mask for. In Krita, I actually sometimes would split alpha, then use cross-channel mask, and set to Destination In mode for fully non-destructive workflow or paint directly onto selection layer then convert to transparency mask. But, it’s not that hard to just put the alpha back into RGB in GIMP.

Yes, as others have pointed out you can brush on black (conceal) or white (reveal) and any grey in between to a mask. That way you can brush opacity, transparency or any combination.