This a list of currently known fusions that will be updated as more of them are introduced. Seriously though, if you can't already guess this has all of the spoilers in it, so read at your own risk. The list is arranged alphabetically.

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Bouquet
Fusion: Rarity + Fluttershy
Aspect: Charity
Personality: Rarity's giving nature and Fluttershy's gentleness.
Magic: Hyper Talent, increases the natural born talents of a pony.

ex. Applejack receives enormous strength and can grow things better

Topsy-Turvy
Fusion: Rainbow Dash + Pinkie Pie
Aspect: Defiance
Personality: Rainbow Dash's mischievous nature and Pinkie Pie's awareness of others
Magic: Unknown, can create very powerful simulations