If the future Mane 5 run into the Mane 6 from Old Equestria and things would probably go like this.

(Mane 5 after they go to old Equestria through a time machine or some magical ancient spell Sunny discovered brings them to when Twilight Sparkle and her friends had vanquished the evils of Equestria.)

Twilight Sparkle: Who are you?

Sunny Starscout: We're from the future, my name is Sunny Starscout, and this Hitch Trailblazer, then Princess Pipp Petals and her sister Zipp Storm and over there is Izzy Moonbow we're trying to find out why friendship is not present in our time thanks to unicorns, Pegasi and Earth ponies no longer being friends!

(Mane 6 are shocked)

Twilight Sparkle: But that's impossible! My friends and I spread friendship throughout Equestria and ponies and creatures are living in Harmony with one another!

Applejack: And ponies ain't friends anymore?

Rarity: Friendship is lost?

Rainbow Dash: And ponies are apart because of horns and wings? Oh, COME ON!

Pinkie Pie: Wait a minute... that's not the future of Equestria I hoped for!

Fluttershy: Oh my! Now can imagine all the wars between creatures (starts to cry) and everypony is fightin' all over again!

Twilight Sparkle: So why did you five come to us for? You should go back in your timeline and I'm about to be crowned Princess of Equestria!

Sunny Starscout: Oh dear! We went too far in the past! We must go further into the future and figure out why Equestria's magic was lost! (Mane 5 go into time machine)

Twilight Sparkle: Equestrian magic lost? Forever? Who are these ponies?

Applejack: I think the new generation will have to rediscover magic or somethin'.

Rarity: What do you suppose what will happen to all of Equestria's magic? And did you see that hunk of an Earth pony? He looked very well-formed.

Rainbow Dash: And did you see one that one Pegasus that looked like me and was almost as cool as me?

Pinkie Pie: Yeah and that unicorn was quite friendly and fun!

Fluttershy: And that pink Pegasus was really nice.

Twilight Sparkle: I didn't know, but if Equestrian magic is going to be lost, we must protect it especially when I become the new Princess of Equestria. I'm sure the new generation will be living in harmony with all ponies and creatures never in disharmony. I sure hope those ponies find out what what caused Equestria to go back to it's roots.