Sources directly familiar with the production tell TMZ ... ABC appears to be seeking someone who's not part of 'Bachelor' nation to replace Harrison for the swiftly-approaching shoot. That means no former bachelors, bachelorettes or their suitors ... no matter how popular they are.

Essentially, our sources say producers now seem to have their sights set on someone without ties to the series -- but who's still part of ABC's talent pool. That said, they did make a run at one alum.

We're told Rachel Lindsay -- the first Black 'Bachelorette' -- was asked to take up the mantle, but she declined. She's publicly suggested they consider FS1 sports analyst Emmanuel Acho -- who's not only Black, but also outspoken on issues of race. Let's face it, that's going to be a major topic when cameras roll for 'ATFR.'

Despite Rachel's endorsement, Emmanuel's rep tells us no one's reached out to them yet.

Fans have also been clamoring for Tayshia Adams -- the second-ever Black 'Bachelorette' -- to step up in Harrison's place. She's been vocal about the show's racism controversy lately, but producers have not tapped her

It's unclear if ABC's specifically looking for a POC for this gig -- but all signs seem to point to that being highly considered. It's gonna be a sensitive episode ... not just because of the Harrison fallout, but a potential faceoff between Matt and Rachael Kirkconnell, who's rumored to be the winner.