Notice the title says what MIGHT be.. Meaning this isn't official..
It was really fun to speculate how they would look maybe.

Marsh:
Mudkip- Marsh would make a horrible mudkip. He's really short, easy to be picked on. Of course back then, before he called him self "Marsh Tomp" He had another name.. But because he did not register until he Evolved, it was not recorded.

Swampert - As Marsh grew, i would imagine him turning into the slacker yet successful college kid. He''s less clumsy and listens to music, despite him always being around water. At this point in his life, he wouldn't get mad as easily.

Belaira:
Torchic - Belaira looked out for Marsh at a very young age. Even though she was also made fun of, she did the opposite of what he did. She beat up any kid that talked against her. She became very determined to protect them both and from then on, became interested in physical fighting.

Blaziken - This will most likely never happen because Belaira hates Blazikens. But it is still a possibility that she will forget her hatred in a journey to become stronger, putting appearances aside and being able to learn more advanced moves. At this point in her life, she wouldn't start fights as much. She is afraid she would REALLY hurt someone so she looks for part time jobs instead to support Herself and Marsh.

This was super fun...
I think all PCAers should think of past and future too