Re: something I've always wanted...

[Karl Kleinpaste <karl@charcoal.com>]

Given what information is acquired when, yes, it's very hard.  If you
want a *Group* buffer indication, the problem is that Gnus has no idea
of group content when creating the *Group* buffer.  All Gnus knows is
the minimalist data point, "there are articles here numbered between X
and Y."

For Gnus to know more than that, you would require Gnus to enter each
group and effectively do *Summary* generation on every single group,
without displaying the summaries, but using the information learned to
modify the *Group* display.

What you want is possible, but it's bound to be alarmingly expensive
and slow, especially when (as you said) you are subscribed to oodles
of forums.