I'm heavily against aliens, with only earth to fight on, or potentially earth and the alien homeworld, it's not going to work lore-wise.
I'd like some new time-aspect, but we have to remember that Einstein already went back in time to mess things up and they've been messed up quite badly. RA and RA2 worked it out quite well, with only one paradox to work out, which is acceptable. There's an obvious plothole in RA3's time travel, so going back in time to cancel the effects of the "first" time travel isn't going to work unless you do something like this (https://xkcd.com/1063/), in which case it's pretty pointless anyway.
But that could be the attempt of an army from the future. They see a devastated planet, go back to join in the fighting, hoping to make the war stop before the devastation. You then also have the paradox-avoiding reason for the time travel. The army from the future goes back in time to prevent an apocalypse but is actually the cause of it.
However, I'm most in favour of something else: another game I'm a huge fan of is Supreme Commander and it's expansion Forged Alliance (in which aliens did come along, but in a better way than they did in C&C 3). Someone with the nickname "dotswarlock" has written fanfiction on SupCom and in book 6 (supcom book 06), he describes a human faction theorizing on the appearance of this new enemy (which turn out to be the aliens):
This time it was Clarke who answered: “If it was just one or two new units then we could undoubtedly trace this unknown force to one side or the other. From what we can tell, however, we are facing a whole new breed of ACU technology. All evidence so far points to the Mayr scenario.”
The idea seemed farfetched and yet this was the only possible explanation. The Mayr scenario was named after an evolutionary biologist called Ernst Walter Mayr. The basic theory was that a faction could secretly send part of its scientists and citizens to the far reaches of space to develop an entire new branch of warfare technology. A society which was removed from the Infinite War in such a fashion could dedicate all of its efforts towards creating an unstoppable army. There was also the possibility that such a society would, in time, develop its own social structure and even a new set of priorities.
Just a link to all the work (with that quote it's not really important though): https://docs.google....U4tODI5Y2s/edit
This would be a plausible explanation for the fourth faction, including the icon. After the allied victory in RA, country-sized colony is developed somewhere in the Pacific (with all the island, more probably, all over the place in the pacific). Although it's very much possible that this has been done by Allied HQ (the units seem to suggest it), this faction became autonomous and adopted a new set of ideals that is incompatible with those of any other faction.
Based on the things I've seen, I'm thinking of Orca Bombers, Disruptor Tanks (I LOVE those, though we already have the basswave and the prism tank) and the Firestorm Wall. That means this is the GDI faction and therefore I want this:
Edited by Plasma_Wolf, 24 June 2015 - 08:24 PM.