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First things first, I will admit that I am not familiar with MLP:FiM.  I will also admit that I watched a couple episodes of the original MLP, but that was so long ago that I barely remember it.  However I do think I can grasp where you're trying to go with this deck so I'm willing to give you a try.

Bad as it may be, perhaps you might like Rainbow Vale ?
Undiscovered Paradise is slightly less craptastic.
Trace of Abundance also has that "paradise" vibe to it.

I know you were asking for spells, but I couldn't resist looking for more ponies (or pony-like creatures...):
If MLP:FiM has a sort of anti-Hero or forever grumpy pony Spitemare looks like a horse and can be cast with just white mana.
Elderwood Scion can be your big pony...although it does look like a moose/elk...
Lucent Liminid
Purity again with the elk thing
Wispmare them legs...but you said you were altering art and it does add utility
Shinewend ok so it's a flying lamb, but that art...

Unfortunately spells of Pony character are a bit harder to come by.  Mainly because MtG is a game about fantasy combat and parties, smiles, and laughter (withouth the "s" on the front) are hard to come by (mad props on finding Planeswalker's Mirth, I don't think I would have).
Conviction I'm sure this emotion comes up at some point, where someone makes a decision and decides to do something about it.
True Conviction see above
Heroes' Reunion a reunion is  sort of a party...sort of
Tribal Unity will probably require an art alteration, but unity is sort of close to friendship in MtG
Honor of the Pure
Debt of loyalty
Pledge of Loyalty closest thing I could find to friendship.

That's about all I've got for ya.  Sorry.
Good luck on the deck.