The Tyranid already tried to eat Ultramar but failed. But GW could always create another chapter make some books about them and then have something cataclysmic happen to there home world and make the few survivors fight to survive without any hope of ever recreating the chapter.
To lose Macragge? Really? That is almost worse than destroying Terra. You have to keep in mind Macragge was constructed by Guilliman basically, Fortress Hera was even impossible for the Tyranids to take over, lol. (Though mostly Macragge has Plot-Armor because Guilliman is there) Baal (Flesh Tearers Home) would be something else or Crimson Fists. Even White Scars. Or those we also know well like Novamarines. Yet it has to simply written right... The Betalis Campaign as written in 'Doom of Mymeara' had it very well done. It was a new Craftworld introduction (I believe) which fought against multiple Regiments and even Space Wolves and Titans! A good loss for the Imperium! (Especially since Mymeara came waaay victorious out of it) The Orks just need a story like that! http://wh40k.lexicanum.com/wiki/Betalis_Campaign#.U_jwHvldXK8
Oh, yes. To lose Macragge and have Ultramar burn would certainly make an impact. Even better, Girlyman's statis cell being destroyed in the process and him being brutally butchered by invading Orks... well, or something else... anyway, it would be glorious. It would add some well-needed gravity to the story of 40k as opposed to the never ending status quo we've got right now. It could be an interesting story to read, if it was well written.
I consider myself a 40K noob; a passionate noob, but a noob nonetheless. I only own Space Marine for the 360, though I have played various entries in the Dawn of War series several times at my cousin's house and thoroughly enjoyed it. I absolutely adore what I'm able to find out about the lore on online resources such as Lexicanum. I'm trying to give this an accurate answer without any hint of bias, and I'm going to have to say the rivalry between the Space Marines and the Chaos Space Marines for obvious reasons. I want to see Orks and Tyranids beating the piss out of each other. Nurgle vs. Tzeentch and Slaanesh vs. Khorne are other rivalries I'd love to see more fleshed out, especially the first.
The never ending status quo will be broken by the Emperor's Death / Rebirth. Or when the Tyranid Warrior Fleets finally arrive.... (As said a few thousand times, the current Tyranid Fleets are only Scout / Recon Fleets)
Thats the only thing that can break Status Quo, besides the Void Dragon suddenly freeing himself from Mars and leaving the Imperium without the Mechanicus and bringing the Necrons to a new height.
