I really like the idea of Archer being Future Guy.
If they wanted it to be Archer, then that's what they wanted. Nothing about what I understand about Future Guy and his influence over the Suliban seems to make any sense at all with it being Archer. On the whole, he's a very decent human being through the Enterprise's missions. Can you see him being responsible for the Suliban? Changing their species and helping to distribute stealth technology? How does he justify that?
Well that would be kinda the point. Something terrible happened, making Archer do these things. Then you'd have the contrast of young, good Archer vs. old, bad Archer. Imagine you meet your future self and recognize that you become the next Hitler despite your best intentions. That's a great theme.
It really would have elevated the show. "Future Guy is a Romulan" simply doesn't have that ring to it. So the main bad guy is from the Bad Guy Empire. Quelle surprise!