What feature do you want to see added?

a way to bind multiple brushes (much more than 2) to one item, maybe
/multibrush, short: /mbr
to turn an item into a multibrush and then a separate command, maybe
/mbr add
to add a brush to the item and
/mbr remove n
to remove the n-th brush (n=1,2,3,...), e.g.:
/mbr add sphere glass 5
/mbr add sphere stone 2
creates a brush that generates a glass ball and a smaller stone ball inside it,
/mbr remove 2
removes the second brush, so it only creates the glass ball.

Additionally with
/mbr n material
and
/mbr n mask
one could change the material and add a mask to the n-th brush (n=1,2,3,...)

In the example above
/mbr 1 material white_stained_glass
/mbr 1 mask air
changes the material of the first brush from glass to stained glass and adds the mask air to just the first brush.

To prevent lagg, there should probably be a maximal number of brushes you can add to one item, that can be set by the server admin.

Are there any alternatives?

You can kind of combine different brushes already by nesting "#mask[mask name][true-pattern][false-pattern]" patterns (see example below), however this forces you to put everything in one chat command and there is a 256 character limit, which is way too low when using modded blocks with long names.

Anything else?

The reason I want this feature is, that I'm trying to make a brush that replaces block A1 by block A2, block B1 by B2, C1 by C2, and so on. (I need this because I am building ruins where I have to create a gradient from one block type to another, so i have to replace stone blocks,stairs,slabs,... simultanously with another material, e.g. andesite blocks,stairs,slabs ... .) I am able to do this with
/br sphere #mask[A1][A2][#mask[B1][B2][#mask[C1][C2][#existing]]]
but again, it forces me to put all the long block names in one line (and I am also using the conquest reforged mod which includes some very long block names!).

In general this feature should allow much more powerful brushes, maybe someone else can come up with some nice examples.

If you think a multibrush tool is "too much", alternatively it would be enough (at least for what I need it for) to add a way to circumvent the 256 character limit in chat or maybe create shorter numerical block ids for modded blocks