While I agree that it would be cool, I personally don’t think we need any more allomancer main characters in era two. It’s just personal preference, but I’d like Steris to stay as she is in terms of allomantic power. I like having that character who is ordinary in a world where there are people who can effectively fly, manipulate emotions and have superhuman strength. This also provides space for her individual strengths to shine, without being overshadowed by flashy powers.
Additionally, her skills don’t lie in fighting or espionage, so while it may be interesting to watch her try to grapple with the powers the come with being a mistborn, I don’t think it’d suit her overall character. A mistborn character would also probably be too over-powered, considering that there are only mistings in era two.