I honestly don’t think that the Elements will be passed down to Luster Dawn and her friends, probably because two of them aren’t ponies.

@Background Pony #158D  
I honestly wish she didn’t move back. That castle in Ponyville is more than enough of a place for a ruler of a nation to stay and even carry out their governmental duties from.

@Wiimeiser  
The buffalo only appeared in one episode though, and nobody ever pitched an idea to bring them back, which is disappointing
 
I suspect that the sixth member would be a changeling, but they could pull an EQG and give us seven main characters so you can have a zebra as well

@CandiCookie  
Zebra, I guess. We already have multiple buffalo.

@Background Pony #E63A
 
I love your headcanon.
 
I had the feeling the yak is Rutherford’s son or relative at least.
 
What about a zebra, or a buffalo for the 6th friend?

It is strange, seeing our old friends, waving to the new herald of the torch… I feel it’s worth asking the question that has yet to pop up though.
 
These four newcomers. If G5 had been a continuation, rather than a reboot… would we have followed this group?
 
That black griffon. Her sash gives the feeling that she’s an adventurous sort. Maybe she prowls the wilds?
 
That green-maned Kirin. Her colorscheme is remarkably similar to Rain Shine. Maybe a daughter, a princess in her own right?
 
Gallop J Fry. Obviously an adult now, what does he do? Is he a line cook at the hayburger place now? And how did he fall in with this lot?
 
That red-headed Yak. Yona obviously lives full-time in Ponyville now. Whats this one’s reasons for sticking around?
 
We’re also obviously short a friend. Who would make a suitable 6th? We have Luster and Gallop, maybe a pegasus…

@CandiCookie  
Twilight did leave Starlight in charge of the school and her castle during the season 9 premier, so it certainly seems like the most obvious solution…

@Ledvi  
Maybe Starlight still lives there.

@Background Pony #7BA1  
No one. Just like Celestia and Luna’s old castle before Canterlot, Friendship Castle is gonna rust away…
 
Or maybe be a tour site.

wait I noticed something in this image? if Twilight lives in Canterlot again than who lives in Twilights friendship castle?

@Ledvi  
yea ^^

@Steel Crescent  
Everything must face their end, including friendship. Instead of worrying about the day Twilight’s friends meet their maker, focus on all the good times she made in her life.

@Steel Crescent  
Well, at least we have 20-30 years of empty world to fill in with whatever we want

@Vinylshadow  
True but its something that dawned on me, just doesn’t seem right after all the good things she and her friends did.

@Steel Crescent  
Cherish the time and memories you have & make, rather than thinking about what’s to come
 
Besides, she’s got a new batch of friends already on the way here, so she’s not going to be alone for long

this ending isn’t as happy as it seems if you dig deeper.  
Twily’s friends are aging but twily isn’t. What kind of reward is that? Someone who basis her world around friends and friendship has to now watch her friends age and die? All the good she did and now she has to watch them die while she keeps on going.
 
Thats grimdark as bleep

It’s so weird to see the Mane Six like this, but things change over time.