I personally think [i]RuneScape[/i]'s lore is terrible and unworthy of an RP adaptation, but the real problem with your idea is mechanical: most people stopped playing [i]RuneScape [/i]a very long time ago, either because they grew up, or (as in my case) because the new management drove the game into the ground with half-baked content, microtransactions, etc. Some people played a while, but eventually grew bored and quit. Others were F2P, or simply didn't like the game, and didn't explore very much of it before dropping out.

I would assume that JaGeX has added only more content since then, so anyone who's hypothetically interested in this RP must be a current [i]RuneScape [/i]player in order to understand the lore.

Maybe design your own setting which is "based on" [i]RuneScape [/i]instead of using its canon directly. Then the players will only need the information which you divulge to them in the thread. This ought to help with recruitment, since it won't demand of the players that they know lore from a game which they may have stopped playing half a decade ago.

Good luck.