I know this is probably a lost cause as they are so persistent with Materia and will probably attempt to fix it by Cross DC travel, not by actually fixing the actual population.
But in my opinion the proper fix is something like this:
1. Move the server to somewhere in SEA region. Yes I know it's unfair for people in Oceania as they will be getting more pings but what can you do... there are more SEA players in only two JP worlds [Tonberry & Kujata] than maybe half of Materia combined. This will boost the population massively and I'm sure OCE players would rather have 50 more pings than to have no one to play with (at least some of my OCE friends said so). SEA players that had been in JP for years are now reluctant to move because they waited to see how Materia fared and it failed... so most stays in JP or transferred back to JP.
2. After moving the server, give SEA players the ability to transfer from JP to Materia for free. You slap the word SEA to Materia and SEA players will flock to the server. Right now they have no reason to. They have a house, they have better ping, and neither OCE nor JP is their actual 'home' anyway, so they might as well stay at the place they've made friends and purchased a house on. Most (if not all) SEA players speak english in JP so language barrier shouldn't be an issue.
That's it. You've just created a new popular DC, potentially the size of EU. And you've also made the game more accessible which would attract more players that are considered new customers (so many untapped potential from SEA, and while it's not the biggest economically, the gamers here love to spend money on cosmetics - ask any Korean MMO and mobile games). But SE is SE and they are so stubborn for mistakes they have made, especially ones that have costed them a lot of money. Materia wasn't well thought out, they didn't do enough research about the market.
I might be biased since I play from SEA. But I would love to play with OCE friends... rather than seeing them suffer, queueing for dungeon for hours. It's just sad.