having the ability to give the masked brush a masked brush would be nice.
I’m not sure if I’ve fully understood what you’re asking for: You can already apply a mask to an animated tip. And use an animated tip as a mask. And even use an animated tip as a mask for another animated tip. You can also already apply a pattern to a masked brush. 
Say i have a square tip (maybe a little transparent) then i give it a masked brush with the same square tip but make it 80% smaller so the strokes is fuller in the middle. Now i want to gice that 80% center a grunge effect. I would need to give it a mask for that.
For that example: You should be able to bake the first part into the primary tip, then you can use the single mask for the grungy bit.
I mean - I’m not disputing your main point: I’m sure there could be some useful applications to having a multi-mask. It would likely come with a performance cost though.
My initial post was because you mentioned several things, some of which sound like things you can already do.
One related thing I would like to see is masks for the colour-smudge engine.
I removed the others things.
Even if i baked it into the brush tip
Playing with masked brush would not be as flexible as having a masked brush within a masked brush.
in that i like that i can change the size as i please and it is still confined (as long as I dont draw my line next to itself, when it bleeds onto adjacent lines).
Unless we can force a cutoff on a brush tip
That too would be a cool feature, epecially since sometimes i like the look when the size is big but do not want it to extend outside such that it hits the tail of my long strokes where the path goes near itself.