This question has been bugging me a bit lately, so I decided to see what everyone else's thoughts on the subject were.
I tried doing a search through the Movie forums here, but I don't think there is a specific topic regarding Homunculi traveling through the Gate. I could be wrong however, and if there is, then I'm sorry for posting this topic
Anyways, if you wish to answer the question, a post describing why you chose the answer you did would be great. Here are some of my thoughts on the subject:
Envy became a dragon either because of the Gate's power (he had no other choice) or he simply shapeshifted into one.
Now if it's because of Envy's shapeshifting, then its probably true that if another homunculus went through the Gate would become humans. Now, the Fuhrer's alternate did happen to be human. However, Ed mentions that at one time there was someone human that looked like Pride. As all human transmutations are done by trying to bring back humans that were once living, this makes sense. Still, it could simply be PRIDE'S alternate and not the once living human that Dante tried a human transmutation on.
If its due to the Gate's will, then a homunculus could perhaps become just about anything! It could very well be that the Loch Ness Monster, Bigfoot, etc. are all the FMA Earth's equivalent to a Homunculi that traveled to the other side. Now, I could see why the producers chose to make Envy a dragon, as a dragon carries the theme of immortality (Envy was the 1 of the 7 that lived the longest), as well as being able to be used as a LITERAL Ourobors that the Thule Society could use to travel to the Alchemy World.
As for my thoughts on the subject, well I'm torn between just about all of them myself. I'm probably leaning more towards the idea that it was due to the Gate's will, but I'm still not sure what, if for example Greed or Wrath would be sent through the Gate, as to what they'd become.
I can say that I am pretty certain on one thing though: whatever form a homunculus takes when they come into our world, their body can be used as a sacrifice for the use of traveling to the Gate. Afterall, a homculus is a product of alchemy, and I suppose that they still contain some form of "alchemic energy" that is necessary for the use of alchemy (or possibley just Gate travel). I'm guessing that the Hagaren world must be full of this energy, while our world has none.