In the Mets own division:
In Atlanta there's Matt Olson, who seems for all the world like a pure firstbaseman, but he spent half his time in the outfield in the minors.
Philadelphia is playing Alec Bohm over there, moving him from third to accommodate Luis Arraez playing second, forcing Brandon Scott to move to third. Bohm going to first forces their superstar firstbaseman back into the outfield, from where he moved to first at 31 despite having played a total of 1/3 of an inning there previously in an emergency situation. He had earlier in his career moved from catcher, where he played in junior college, to center, to right, to first, and now back in right.
Miami's most frequent occupant of first base this season has been Kyle Stowers, who has spent all of nine out of his total 3,700+ minor-league innings at the cold corner.
And here we are playing Washington, breaking in a brand-new first baseman, a converted outfielder who is not situated where he is because his experience and nuanced dexterity make him a fine asset. He's there because he's hitting like a mofo but he's too hefty to put anywhere else.
Folks don't play first because they're good at and experienced at playing first. They play there because they can hit and lefthandedness and/or defensive limitations relative to the other eight guys make them poor choices to play elsewhere.
The Mets had the best defensive firstbaseman in history, but when his bat stopped being effective, his glove couldn't stop them from getting rid of him, and Cleveland quickly realized they were right.
The Rays have baseball's best record, and they're playing a guy at first who was a middle infielder up until he reached the cusp of the majors.
And then there's Scott Hatteberg, whose experience concerns over his move to first were so overblown, somebody made a movie about it.
So yeah, when looking for someone to play first, I have no problem with considering everybody to be candidates. Guys transitioning from third, catcher, shortstop, weak-side linebacker, goaltender, fishmonger, lumberjack ... if they can hit, they're a candidate to play first.