I have been racking my brain to find ways that ROI could bring in fresh blood (and possibly older players who hadn't seen the new updates). I know to some this might seem like a futile effort, but I'm determined to try to help so that another one of my fav games doesn't wither away into a pile of dust. I know this will probably take some time and money, but it might just help restore some players to the game.
Youtube/ Twitch ads: probably no longer than 30 seconds or else people will just hit the skip button; make sure the ads have action shots of Riders in dungeons or taming, and maybe mention adding a new equipment, new familiar, or new location right before an update
*Similarly, you could also start putting up a Google ad or apply for banners on gaming sites like Kongregate
Youtube/ Twitch users: get some well known Youtuber's and Twitch users to play and review the game
Offer incentives for joining: this would be in the form of a banner that offers a certain percent off their first ellun purchase, or access to a familiar box
Sell products: You could sell plushes of various familiars along with posters, bags, pencils, pretty much anything that a player might want. From there, you could host giveaways and even offer codes (golden ticket style) for improved equipment or tours of the headquarters etc. (Familiar trading cards, anyone?)
IRL Challenges: This would probably be more social media based, but you could offer challenges like trivia challenges, raffles, giveaways to people who fall into a certain criteria (tame the most of one type of familiar ie wolves, horses, dragons), etc. I know there is something similar in the forums, but remember, most people don't really know about this game or assume it's not as great as 'X' game.
Add a 'fun' server thing?: This would be where players could do familiar racing, slots, hide and seek style games etc for a few rewards (top player gets gold or boxes with good loot)