I guess this could be a plugin, but it seems like something that would work better if it was integrated into the interface... anyways, how about adding a queue for all the resource-intensive jobs? I often find that I end up opening a lot of windows when I'm replaygaining my files, but opening too many tends to choke the program.

Since processing everything simultaneously probably isn't gonna change much in terms of speed after a certain amount of processes, why not make a universal queue for everything? Every time a plugin wants to do something, it'd add an entry to the queue. Jobs in the queue could be moved around in the same way that playlist items are currently, but the queue itself would be in a seperate window. This way, you could control how many jobs are going at once for any given task, easily cancel jobs on specific files, plus a bunch of other functionality

Oh yeah, and Album Gain and other similar group tasks would have to be grouped as one item, but individual track gain on a group could add one item for each track.. just something else I thought about.