[@Warborn123] 
I don't know that "Escort the McGuffin for forever and a day" would really work as an Only War plotline. I could see it working for Rouge Trader, Dark Heresy, or Black Crusade, but with Only War our PCs are otherwise inexorably attached to our Regiment, and the PCs aren't getting off-world on their own.

[@AdvancedJ3lly]

Ork Waaaagh, attempted re-taking of a fallen Forge World, defensive action against a rapidly awakening Necron Tomb...

It seems like Drop Troops (be they Elysian or otherwise) are the consensus as to the Regiment choice, which could open with the cavalry arriving in a massive battle and turning the tide, then segueing into overall siege warfare with our regiment acting as a strike and recon element.