Another thing is that Izumi mentions that letter Dante wrote is dated in the A.D calendar system something not in use for 400 years. It could be that the world across the gate is the past and that Hohenheim somehow is the person that originally develops alchemy and creates Envy when Ed dies in accident. That would be a really messed up time loop.
They really left the ending open for a movie or another season.
Some of you might find this interesting particularly the dates:
I have to go to class now but it's interesting trying to figure out if there was really a group like that. In Edinburgh, Germany as well as London there was a Temple for The Hermetic Order of the Golden Dawn or the Alpha et Omega.">