[Guido]

It is unlikely that this going to happen, but actually I was working with the following fanfic speculation: what if there's a possibility for Arakawa Hiromu to introduce new Homunculi aside from the already established seven in the manga?

My answer is both yes and no, and let me explain you why.

For this to happen, Arakawa-san should extend the plot from what he originally foretold in the manga. As it is important for him to explain the role and relation of this unknown Homunculi would regard to the preconception of the Philosopher's Stone and the Elrics.
However, the flaw on this relies that there's not enough time for the anime to do that. I mean we're just 18 episode from the finale, and that leaves neither time nor space to develop new characters aside from the multiple cast already settled.

Nonetheless, it is possible for Arakawa-san to introduce new characters this far, at least in the manga. Because he wants to continue working on with the manga, after the anime has ended.

The only possibility to work this fanfic theory for the anime is if the show should be extended at least 26 episodes from the already 51 budgeted. Something that won't happen due that Sony announced that FMA only will last for 51 episodes.

Another important aspect for this happening is that the introduction of said Homunculi should be done in any manner that will not disrupt the flow or progress of the plot whatsoever. In order to do that, these Homunculi should or must be hinted at.

What I mean is that if the crew would not like that new character's role and introduction seems farfetch'd to the audience, there has to been time and the right moment to give hints of them and give them fair treatment before introducing them. However, I say it again there's no time for the anime to do that unless the show is extended.

There's also the matter that the plot clearly has stated that only seven appear. However, the plot hints that all those seven were created from sins of people who transgressed the laws of alchemy: The Elrics, Izumi, Dante, and others to come. Thus, their names taken from the deadly sins.

Another possibility for this to happen relies in Pride. He/she/it has appeared neither in the anime nor in the manga. This leads to a nice plot twist in which I could speculate that Pride knows about the existence of other borned Homunculi which the other six may not know of. This said Homunculi could be either waiting on the other side or being summoned by Pride, because he/she/it could be aware that they play an essential role to the Philosopher's Stone preconception. There's also this fanfic theory proposed by me that could turn out in an awesome plot twist and lies if it was Pride himself/herself who created said Homunculi or born from him/her, with the help of an important Alchemist, (that you maybe aware of) of course.

However, I say it again, there has to be enough time for this to happen, so that these new characters can be developed at their full extent.

The Homunculi that I'm treating for this fanfic discussion, I put their names from the four signs of the Apocalypse (the riders).

Death
Plague / Pest
Famine
War