The next tier 1 is going to be ARM, probably for both Android and Linux, and then I personally don't see any others on the horizon. I actually see our tiers being redefined soon to be more graduated, and not have so many tier 2 commitments (we're happy to build artifacts for obscure platforms, less happy to be on the hook for guaranteeing obscure platforms continue to build - not saying FreeBSD specifically is 'obscure').
That said, the differences between tier 1 and tier 2 are not so great. If a platform has testing on CI then it is pretty close to tier 1, even if it's technically tier 2. The problem here is that testing the FreeBSD targets requires running FreeBSD in some way and that has been hard for our CI to do. Anything that can be crossed from Linux is easy, everything else is a painful special case. So with FreeBSD we do the the cross-building, but not the cross-testing. So the most effective thing to do to take Rust FreeBSD support to the next level is to figure out a way to get the CI to run tests.
There may be opportunities in the future for paid Rust platform support, where if a motivated party donates the right amount of money, we'll make it happen. I think that will have to happen to get some of the remaining platforms up to production quality.