I'm sure we're all familiar with the idea of speaking with our wallets. Suppose my favorite character in some live service game was a generally less popular character. As a player, what else can I do to nudge the game developers to release skins or other character specific items so that I could speak with my wallet?
Unfortunately, there really isn't much you can do realistically if your favorite isn't as popular as the others - all you can really do is maximize your engagement and hope that it's enough. Let's go over the levels of engagement you can do if you want to show the developers that you want more stuff for this character. Engaging with these will absolutely not guarantee anything, but will help optimize and increase your chances of seeing more content for your favorite.
At the very base level, you need to play the game and use that character a lot. All live services keep metrics of play, so we want to see which characters are used and how often. You'll get bonus points if your favorite character isn't top tier in comparison, it shows more interest.
Beyond the base engagement layer, if possible you should be spending on that character - on new content especially. If you have the budget, buy skins, emotes, stickers, whatever. Remember the old adages - "talk is cheap" and "put your money where your mouth is". If you want to convince the developers to make content for a character, you need to show that there will be people who will spend on it.
Engage with character content both in game and outside. Be a character evangelist and talk with others in the community about the character. Get into the details in public conversations, show the devs that you care and are interested. Try to persuade others to think about things. Get some healthy discussion going.
If you already spend on character content and engage both inside the game and outside through official and unofficial channels with character content, then that's already doing as much as you can really hope for. If you really want to maximize your chances, you can make your own fan-created content - fan art, fan fiction, cosplay, user-generated/mod game content, and so on. Cosplay especially goes over well - it requires significant dedication and craftsmanship and it shows well to help sell the game to outsiders too. It also has a higher level of virality than fan art or fan fiction - people are much more willing to share cool cosplay pics.
Finally... be willing to accept that doing all of this probably still won't move the needle very much. When we're building content for a game, there are generally a lot of other factors at work beyond popularity that determine what we choose to include. We need a good amount of content coverage because live service player bases tend to be a bunch of various partially-overlapping sub-communities. As we build content to release, we distribute what we can to each kind of sub-community to keep them fed. We do what we can with what resources we have, but the thirst for new content applies universally.
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