One Hearts-&-Hooves-day, Princess Mi-Amore-Cadenza, or Cadence for short, decided to help every-pony become closer with there special-some-pony, so she uses a spell to help all ponies become closer with there special-some-pony, but some-how, either she miss read what the spell was, she miss casts the spell, or Discord did something, ether way all ponies did become closer with there special-some-pony, they all fused with there special-some-pony !

Shining-Armor with Cadence, Lyra with Bonbon, Octavia with Vinyl, Spike with Rarity, Braeburn with Little-StrongHeart, ..., and Celestia with Discord
p.s. or what ever your ships are.

Big lols, especially in cases of teenaged love triangles and weird couples.

4977890 Oh YA !

Love-Triangles: the "stallion" gets split in half, and each half fuses with one of the "mares"

Friends-With-Benefits: They become "Conjoined-Twins" if you know what I mean.

polygamy: All "3" (or more) gets fused together.

p.s. What do you think of this ?

4978781
In all cases, I would imagine ponies becoming conjoined rather than merging completely. Not such a high quality drama unless there's some odious physical discomforts and/or personalities get to rub each others raw.
As to the love triangles, I imagine the three conjoining into a tricephalus body, with the object of affection being stuck in the middle of the two rivals.

4980096 MHM... what about like FireStorm ?
Like Ronnie Raymond & Martin Stein

As to the love triangles, ..., with the object of affection being stuck in the middle of the two rivals.

What if the two rivals are like the neck & neck, (like the tiger & the goat) and the object of affection where the snake is