I don't know for certain if this type of concept exists but I'll take a stab at it anyway:

As we all know Princess Luna became Nightmare Moon because she felt her nights were not being appreciated. Ancient history now.

We learn in later seasons that not only Princess Luna has the ability to Dreamwalk(entering the dreams of others) she also possesses the ability to Dreamweave(the ability to fabricate a dreamscape) and Dreamlink(the ability to link the dreams of multiple individuals to each other allowing free travel for those dreamers, like Pinkie Pie jumping from one pony's dream to another).

I propose this concept, Princess Luna's greatest achievment if you will:
By combining the 3 previous abilities: Dreamwalking, Dreamweaving and Dreamlinking what if Princess Luna created what could be considered a Dream City. Slumberland if you will.

It could be used by anyone who can dream(pony, human, etc) across both Equestria and Earth as a means of interacting with each other while they are sleeping.

Different sections of the city could be tailored to the needs of different types of individuals. For example, foals and children would have their own dream version of a theme park to have fun(think Dream Disneyland/World but without the hour long lines). Or maybe the more studious of us need a place to research(I'm looking at you Twilight), so a section could be dedicated to that with all of both worlds books copied into the Dream Library. There are so many possibilities.

It is in essence a hub world for dreamers, limited by only the imagination. Feel free to claim this idea as your own.

This idea was inspired by pieces of fiction like Ready Player One:

So something like Lovecraft's dreamlands?

6333780
In a sense. Best thing is that it can work in cannon too.