I know that expansions in each and every MMORPG are rare, and that ideas for content are probably the least of worries in the process of coming to a new expansion, but I’d nonetheless like to draw your attention to the works of the Argentine author Jorge Luis Borges.
No other game could better match, and do justice to, the imagination of Borges that Secret World Legends could. His stories usually deal, in some way, with infinity, and with dreams, labyrinths, libraries, mirrors, fictional writers, philosophy, and religion.
The idea that could most probably be best implemented, lore- and location-wise, is The Library of Babel. I can imagine this to be some sort of a hidden annex of the Sunken Library — or an even more elusive counterpart to it (btw Borges’ description of the library is written as if it is obvious, but when you try to reconstruct it, you notice it is not entirely correct, or incomplete. So, an implementation would always be an interpretation. I could image it to be somewhat Escher-like.)
The fantasias of Jorge Luis Borges display a marked resemblance to some of Lovecraft’s more dream influenced work. Borges also dedicated his story, “There Are More Things” to Lovecraft, though he also considered Lovecraft “an involuntary parodist of Poe.” [source]