One of the speculations for RoM (that, of course, seems not to be true as far as we know) was that unlocking the magic would unlock a kind of playable overlay for magic sims that other sims can't see (they way only the child could see their imaginary friend in TS3, or only toddlers see the cool animations on their playset in TS4). I do think that would be a really fun idea to transfer over to a fairy pack, as "fairy sight" or "fairy ointment" is a part of so much lore.
So the pack could include a small new world with everything looking really big, like suggested above, but more importantly have a really in-depth playable overlay for all the existing worlds. Maybe objects would look different to fairies (like, idk, all bars now have a clickable "fairy bar" on them for ordering fairy drinks) and fairies could see other fairies as fairies, and maybe also see like auras/overlays for other occults or for sims moods or something, but you click over to a regular sim and everything looks normal.
I think this would also allow the pack to "invade" the world without invading the world for players who don't want it to, because if you don't play a fairy none of that stuff is there.
But I do like the idea of a magic pack where the magic just kind of layers over the worlds we already have. Probably unlikely, as I imagine it would be a lot of work to go through every world like that, but it sounds fun to play. Especially playing a combination home, where someone is "playing Fairy Bride" (marrying an unknowing mortal) and trying to keep the secret.