Just having played the first mission "Fort Clarke Assault", I thought it was great, even tho it still felt a bit empty compared to the official campaign because it had no transmissions or voice acting at all.
Maybe an idea to make it a bit more lively would be to have the Seraphim listen in on some of the transmissions you would normally hear in the campaign (especially the death transmissions when failing) or just some of the Seraphim threats.
This would have the advantage of massively reducing development time by using already given elements and letting the player experience some of the content, they would normally not get (assuming that you won´t fail every campaign mission).