as I see it there is 2 main reason ppl are less active in game
1st: unactive staff, no weekly events, no GS that help daily with stuff and questions, and last update was before Christmas.
2nd: is pvp, the pvp event system is awesome but there is a massive difference between old players to new, old players can build a ton in a few days and make it over power and control pvp, this is very frustrating, just think of someone building a toon for 4 months and getting "bully" by a 2 day faction.
there is not much u can do about it, but maybe make a event for the non OP ppl.
a lot of ppl rage quit cuz of seira/hydran, I don't talk about that, cuz u can counter them, need to work hard, really hard for that, but possible.
Also there are items in game that are hard to get, almost impossible due to fact u need more active ppl, such as Buddha's Relic and other Necky, maybe add different ways to get it, like a boss in Elysium that drop it like GoR but not him.
One last thing, the game has lost some great factions, since u need seira/hydran for milky, a lot went to that, and faction like Balo, Celan, Kytos are rare. Even in pvp there is like 2 lupin players 1 modo and rest are seira/hyd.