I'm thinking that Limbo is this place that we ought to develop a little bit. Let's brainstorm some ideas!
Like, I miss all that crazy stuff from Spelljammer, which I think got somewhat integrated into the 4e Manual of the Planes… I like the idea of Githyanki and Mind Flayers and Beholders having weird mercantile empires, on these fish-shaped boats powered by magic-users and clerics…
And like an Embassy of the Abyss where instead of waiting rooms with bad magazines, there are waiting rooms with torture devices…
Oh, and definitely a lot of immigrants from across the TSR Published settings, including samurai and ninja from Kara-Tur, a couple Tinker Gnomes from Krynn, wild-eyed Thri-Kreen from Athas, Shi'ar and the Holy Brotherhood of Slayers from Al-Qadim, and so on… (Not to mention all the "vanilla fantasy" guys from the Realms and Greyhawk.)