I would like to know what can be changed in the Broodwar campaign for a mod.

Aspects I know:
-the displayed planet pictures
-the text display before briefing room
-the map itself can be replaced
-campaign screen animations (replacing Artanis/Kerrigan/DuGall)

BUT can you mod...
-campaign size (count of maps) ?
-which mission leads to next mission (e.g. adding alternative maps where you want them to have (like in the Terran campaign))?


What can you do about that with normal modding + current plugin knowledge?

I'm not 100% sure about the amount of maps, but I think the only thing that limits the amount of maps is the mission select part which will only list the amount of maps from the original campaign. So having less maps would be fine but if you had more you wouldn't be able to jump to the last ones.
For which mission leads to the next mission, its all controlled by triggers, so you can do whatever you want, though like above im not 100% sure how the mission select screen will work.

Plugins can do ANYTHING, it just depends on the persons skill in reverse engineering. Literally you could make SC into Counter-Strike. Check out modcrafters for some examples and info on plugins.