Do you ever do the PTR? I'm not really sure how they decide what accounts will be flagged for beta, but I played a ton of the 2.4 PTR back in Burning Crusade (contributing screenshots and information for WoWWiki at the time), including filing lots of bug reports.
Since then I think I've gotten into every beta/alpha Blizzard has done (every WoW expansion, Hearthstone, Overwatch, etc.), and I've always filed bug reports and posted constructive feedback on the test realm forums. I'm usually not in the first round but I always get in fairly early.
I would suggest maybe spending some time on the PTR in the future and just doing actual testing/feedback, that might get you on some sort of shortlist.