I'm running a dungeon in which the party managed to teleport to the location of the BBEG right after he retreated to his home. Problem is, I was planning to run this part of the dungeon three levels later. I plan to force the party to retreat and probably go back to the story again, but in the meantime I need help devising interesting encounters/rooms. The BBEG is a master pyromancer (Evoker wizard) who dabbles in alchemy. 

My current ideas:

A xorn he was experimenting on, in a magic cage.

A huge glass machine with reservoirs of various chemicals.

Three mixed potions in a closet.

A demon he summoned to guard the hallways.

What level are the PCs, and what level is the expected dungeon level? Also, what was it that they made use of so they could teleport in?

Sorry about that. PCs are level 15 and the dungeon is supposed to be 18 or 19. They used a teleporty artifact they got

Hm. Anything intended to be a dungeon for 18th level characters kind of needs to be full of insane nonsense, such as a portal to the plane of elemental fire (doing continuous fire damage in the area) guarded by some iron golems (which get continually healed by the fire), or a phoenix egg held in stasis that hatches in a cataclysmic fire storm when opened.

I'm running a dungeon in which the party managed to teleport to the location of the BBEG right after he retreated to his home. Problem is, I was planning to run this part of the dungeon three levels later. I plan to force the party to retreat and probably go back to the story again, but in the meantime I need help devising interesting encounters/rooms. 

The PCs are in some sort of hall & notice several open doors and all have interesting views in them like what you listed.  You can even have the BBEG sitting at a desk watching/receiving a briefing.   The party goes through the door, (any door) but the door is actually a teleport portal.   So instead of being in the room, they are elsewhere, aka where you actually want them.  If they have that artifact, and reuse it, they are back to that same hall and see the same images.  That artifact is not actually an artifact, but a "trap" for the PCs.  A harmless trap (Hit point wise) but gives the BBEG a lot of information.  

I guess. I've already had the artifact display cool powers besides teleporting though.

Now that last one is kinda crazy and I think I may use one or both. Thanks!

"...but gives the BBEG a lot of information.  "

The information is e.g. when the two meet, he is buffed so the PC spells are not as effective (resistances?) The BBEG knows the PCs resistances as well.

Another information aspect, is the mages are robbed of all of their components. (BBEG had thieves find them and steal all components and even the pouches)  The "rare/expensive" or even common components just can not be found.  (this can be a delay tactic because the PCs have to go elsewhere to find the components.)  Even be bold enough to say that stores have the components they do not want, but some purchased the exact ones the party wants.  Let them stew with that information.

Basically actions have consequences, they went in half cocked , where found out and the consequences are not deadly, but very, very inconvenient.

As a matter of DMing philosophy, I would not punish the PCs for rushing ahead (though my tier 3-4 bad guys use hallow, mordenkainen's private sanctum, or forbiddance to prevent teleport shenanigans), I just wouldn't necessarily pull my punches other than maybe capturing the PCs instead of a TPK.

It's not going to be a TPK, I've made sure of that. The capture might happen but the teleport artifact can allow the party to escape a TPK. (At its wielder's own risk, of course... 😈)