I know, SMs are already catered for loads but realisticly what do you think could be the large model for an apocalypse size Space Marine army, excluding aircraft?
This. Space Marines don't, or rather can't, fight standard wars like Guardsmen. I read somewhere that the Imperial Guard are the hammer and anvil of The Emperor, and the Space Marines are the scalpel. They're made for fast and hard hitting actions that devastate their enemy with as little troop commitment as possible. The Land Raider and Thunderhawk are as big as the Marines need. Giving them their own giant unit would be a really stupid move in my opinion. Now doing it for the Space Marine legions is much more believable.Nothing. The Space Marines' way of war doesn't benefit from using huge war machines as the Titan Legions and Imperial Guard fulfil that role in a far greater capacity. Space Marines are themselves an elite detachment, not line troops, so outside of Thunderhawks I can only really see Stormbirds and orbital support as viable options.
It's not a stretch that they might have a fellblade in mothballs, but that is just a modified baneblade left over from preHeresy time. They shouldn't have a new SM specific superheavy though. I think SM players would be happy enough with a plastic TH.A large model is not against the spacemarine way of war so long as it is not too big, the spacemarines used to operate in large numbers and with super heavies and then adapted to the surgical strikes after the heresy when the legions got broken up into chapters. Surely it would not be too much of a stretch fluff wise that the marines would still have sum in storage to be pulled out in dire times so long as they keep it on a 60mm base if its a walker