1) There's always a way to bring in a new toon or rework a toon to completely change the meta, like bring in a toon like Mother Talzin that reflects counter attack damage back to the counter attacker (or better yet, she can apply a buff to one or more toons that does so).
2) Rex is always meta. Or always anti-meta, at least. My current team is Rex, GK, zeta'ed FOST, CLS, and Thrawn. Other toons would fit in FOST's spot, depending on your shard, like Raid Han, Ezra, well, pretty much any utility attacker or tank. Ezra, FOST, and Baze are especially nice for their dispel abilities, and FOST giving defense up to the team is very helpful in surviving the current meta.
3) I'm with you. I built up my First Order team months ago, and my Resistance team now, so that I'm ready for any new toons to come. First Order may not even be a meta faction, but maybe Snoke goes really well with FOTP or Hux pairs nicely with FOST. Who knows?