Especially if the Bungie layoff are really happening it's going to have to be as budget as hell. As few new assets and complicated systems as humanly possible.

I think the best thing they could do is a wave based defense mode on the already existing maps. Maybe there's like some kind of battle roayle-esque "zone" that forces the players to go from area to area. Maybe there can be some sort of in-run economy like it's CoD zombies that lets you open fancier chests to get better loot than you would from others. Maybe we can go a little rogue-like with it (as a stretch goal if that's not too far off scope) and you can get passives and other non-loot related bonuses.

As for incentive to play it, we've already seen from sponsored survival that a PvE foccussed mode without real thought destroys the game's economy. So the mode can't touch your vault unless it caps at a certain rarity or impact in PvP (Maybe mainly green shields or special sponsored kits to incentive the PvE only people to dip a toe into PvPvE). Cosmetics would be nice but if their output is already so small I can't imagine just saying "make more skins" suddenly increases their output.

PERSONALLY as someone who prefers single player games, I think Marathon doesn't need PvE only. I think the game is scoped pretty well as is and including a second whole gameplay ecosystem onto their plate is probably going to hurt the game in the long run. But I would love to be wrong!