I downloaded bex's past 3600 tweets through the public api and it only goes back to march 2019 so the only real evidence we have to look at is the legion teasers before she went on leave. Here's what I have:
Legion released Jun 6 2019:
-We are just over two weeks away from announcing our next Path of Exile expansion (May 21st PST). I can’t wait for the carnage of teaser season! (Red herring that started the conspiracy) Mon May 06 21:12:38
-I am very much enjoying the Path of Exile expansion name conspiracy. Here’s the list to guess from if you’re not following along. https://t.co/XO6Z2sQOVw Wed May 08 20:57:55 (THE LIST WHICH WAS THE COLOR OF THE LEAGUE)
-It’s the beginning of a new work week and I’m glad to be working on next week’s expansion announcement and teaser schedule! I’m sure it won’t be long before the legion of Path of Exile fans figure out the expansion name. Sun May 12 21:22:45 (sly use of legion)
-You know what’s weird? I’ve been looking at the Path of Exile expansion name list for years and I only just noticed it’s purple. Fri May 17 21:56:44 (THE COLOR REVEAL)
Correlation of bex's recent post below: theme idea Leviathan or Maelstrom???
Feb 12th - This week I've been able to take a deep dive into the upcoming Path of Exile expansion in preparation for teaser season and I think this one is my all-time personal favourite. You'll be able to make up your own mind, of course.
Feb 14 - Woke up thinking about what it would be like if octopus lived on land. You’d find them under bus stop benches and in your shoes. Maybe they’d fly? What a world that would be.
Feb 17 - A list of things people have guessed for the next Path of Exile expansion theme: -Trees
-Underwater
-Delve but up
-Nemesis? Lol
-Dwarves
-Rice
-Ducks
-Dementia
-Orange
Some of you are close! Some of you could go further.
Feb 17 - When you encounter other devs on your testing realm:
"Ah, wertahert, we meet again. Yes, it is I, asdadafaf." ( this is a stretch where wertahert almost sounds like saying water underwater?)
Feb 18 - aquaman gif
Feb 20 (1 hour after this post) : It’s always nice when people think you’re teasing one thing and then the tide turns and they think you’re teasing another.
Edit: sorry bex
Edit 2: added recent tweets for upcoming league
EDIT3: NOW SHE"S JUST MESSING WITH ME https://twitter.com/bexchangewords/status/1230621147001049088?s=20
edit 4: formatting