For big events like this, there should be a in-game interface with a opt-in/opt-out feature. You can opt-in to be warned of Tequatl’s arrival. Once it’s time for the event, you get a popout in the corner saying “Tequatl is here! Join the Vigil in the upcoming fight!” and you have 30 seconds to click “Join” if you want to participate.
Once you click join, you get teleported (with your party if they wish) in a 100,150,200 people (whatever the number you want) instance until it’s filled up — it needs to be cross-servers too. Then you have a 2-3 minutes timer to pick your combat squad in a new interface : Western Turrets Defense, Eastern Turrets Defense, Attack Group A, Attack Group B, etc. Once you’re in your squad, you can see the clear objectives of your squad (like — Defend the turrets from the incoming risen.) and you can see their dots on the map too (it would be great!). Put a cap on different squads as well so we don’t end up with 100 people in Western Turrets Defense. It doesn’t prevent someone from East to run to the West group but I mean, it gives a little direction to new people upfront without requiring voice chat (which the game itself lacks and we must rely on third party software).
Once the timer ends up, the fight starts, with a full map of people that took the time to click join (so it weeds out some afk people), that are divided into groups with specific objectives and are overall inside a group that stands at least a chance at completing the event.
What if someone wants to join after it started? (say, someone logged in game 5 minutes after the start) Keep a “late arrival buffer cap” in each instance of 10ish people. That cap is exclusively for that kind of people to give them a chance to join a group that stands a chance even if they’re late. Or just don’t give them the opportunity to join. In any case, don’t waste people’s time by letting them joining an overflow lacking enough player to complete the content.
What if my whole guild want to participate together, parties are limited to 5-man teams! It’s time to add a new guild feature where the appropriately appointed guild members (via the current ranking system) can click join for his whole guild (and then all members online can confirm that they want to join as well). So they’re teleported together in the same instance (as long as the guild isn’t too big).
Once the event is done, since it’s an instance, you need to click the little house icon to get out. Yes however, we lose the whole effect of “Sparkfly’s coast is full of risen mouths exploding in the face of people.” but seriously, it’s not an element that makes the encounter fun, it’s only aesthetics. It gets rid of afkers that stay in the map after the event is done since the next encounter would be in a different instance.
TL;DR: My main gripe with the new Tequatl is that we need to camp the boss (YES! CAMPING a boss in Guild Wars 2… I can’t believe it myself) to be able to have a meagre chance at completing it OR be in a very organized big group (even then, they face many problems with logistics themselves as well I’m sure). I don’t want to waste long hours just waiting in-game to not be stuck in destined to fail overflows.