This is a pretty simple suggestion, but it could be very fun I imagine. Now that we have a nice collection of bosses in DoomRL, it would be nice that during a boss fight, the level music changes to certain a boss tune (configurable in the music.lua).
So this music change would happen if you enter CC+, you pull shambler's lever, the bruiser brother's wall lowers, you open AoD's door, and agony/lava elemental appears. When they are killed, the music changes back to the original level's music.
How about that? I think it'd definitely make the boss fights feel more epic if we find a good boss music (final_showdown is maybe a bit too... too final showdown'ish for not-final bosses).