@risa The only requirement is that you haven't had a subscription after May 13th, it'll last until the 26th.
http://www.playonline.com/ff11eu/campaign/wcb/index.html
If you log in before sometime in June, you'll get a Mog Kupon to claim a 1st prize Bonanza weapon(!!), up until now they were egregiously rare since, obviously, you had to get 1st prize in the Bonanza Lottery.
Here's a list of weapons, they're all pretty decent. I went with the WHM staff since it alone caps Cure Potency I, will do me good until I gear up properly.
https://ffxiclopedia.fandom.com/wiki/Kupon_W-MOGB
As for Carbuncle, it's as mentioned very low population, only a few hundred players online at a given moment. 427 people right now.
However, because of that you tend to run into human interactions more consistently, just not as often as Bahamut.
I like it a lot, people are super nice here.
I came here from Asura which was incredibly busy and also full of botters and mercenaries. Bahamut I hear is a better server for those who want a populated server, but with a more casual environment.