Hoy burks! 
What is your view on the World Serpen Inn? I red about it some while ago after I bought a used boxed set of Planescape and I can't figure where to introduce the famous inn with Sigil already existing..
It said that it can be a kind of replacement of the Astral planes but by doing that, you flush out the wonderful race of the Gith Kin.. Do you have any good suggestion or views about this matter?
Greetings, Fellow Canadian. Would you like some of our free drugs, eh?
Just kidding, welcome aboard. I think you're mixing your roleplaying game slang when you say hoy burks (it's hey, berks), but that's ok.
The World Serpent Inn is a good introduction to the multiplicity of the planes without worrying the players (or yourself) with the metaphysics of the astral and ethereal planes. It's a starter location, not a replacement for the planescape cosmology. If you and your players are completely new, use the World Serpent Inn to see if the players like planar adventuring. If they enjoy, introduce the more complex astral/ethereal/inner/outer stuff.
As a minor side note, this section is for campaign stories. If you have more questions, direct them to the RPG Discussions sections.
cheers,
*gets a broom and sweeps the conversation to the right spot*
IMC the World Serpent Inn still exists (even with the introduction of the Astrial/Ethereal/Ordial Planes). However, the Inn doesn't connect with Sigil in any way (thus avoiding the sticky situation where a power may try to exit), but follows a random pattern of appearances at the gatetowns on the Great Road as well as the other Prime Worlds.
Any portal from the Inn to the Cage simply wouldn't work for a portal. I don't see why the situation is any stickier than any portal elsewhere in the planes.
Actually, the Inn was introduced before Sigil was presented in the Planescape game setting. The Inn was to be used with Advanced Dungeons & Dragons general 1stE setting as a means for planar travel for those who couldn't (or didn't want to) handle the complications of explaining (or using) the Astral or Ethereal Planes.
A portal is a portal is a portal. If there was a portal to the Cage, it would work as similarly as a portal from the Cage to Sekolah's realm in the heart of Sheyruushk or a private kiln of a Dao on the Elemental Plane of Fire. The Game Master sets the restrictions on how the gate will work, under what circumstances it misfires or fails altogether. There's no logical reason why a door in the Inn couldn't open into a room in the Bariaur Inn.
I referenced a "sticky situation" for those who might attempt a bum rush (or sneak attack) on Sigil by unloading powers through a Sigil exit in the Inn. Powers are restricted from Sigil and that's that; they are free to roam every where else in the multiverse (in theory) without consequence or restriction. Depending on how your campaign is structured, you can restrict them however you see fit.
That's true, but I believe it has been referenced in at least one Planescape product as connecting to Sigil. I can't give you an exact reference, though.
I was trying to figure out what your point is, but then I realized the mistake was mine.
I mistyped my comment above. What I meant to say was that any portal from the Inn to the Cage simply wouldn't work for a deity.
Not all portals work the same way (the portals in the gate-towns can be particularly weird, like the one that's a bariaur or the one that's getting lost in the woods), but that wasn't the point I was trying to make here. You're right that a portal in the World Serpent Inn would probably be similar to a typical portal in Stygia.
That just wouldn't work. The Lady of Pain controls all portals leading to Sigil, and gods can't use them.
Most others could, however.
I think it has portals to both Sigil and Arabel in the Material Plane by 2nd edition sources.
It's different from Sigil in that gods can visit it. There are also more links to places that don't, properly speaking, exist. Or shouldn't.
I wouldn't ever consider replacing the Astral Plane with an inn, even an infinite one.