Due to the way RSII works, it is possible to easily create a new campaign without altering the existing files by creating a new modfolder.
Doing this, you can without too much hassle create brand new campaigns with new factions (such as swap-factions, or new ones created by you with new units, ...) by copying the content of an already existing mod-folder, and modifying in it what you want.
Is such sub-mods do appear, I'm able to edit the launcher to make it that your new campaign is choosable there. If the need arise, please contact me.
Thanks, Paedric.