I always thought Sparkle ponies with twinkle eyes would be cool! Add tinsel hair and a jewel symbol, and you get the sparkliest pony ever!
So Soft G1 babies would be darling!
Scented twice as fancy ponies would be nice too, especially a berry-scented Sugarberry or candy scented Yum Yum.
Not quite a gimmick, but unicorn and pegasus Big Brothers!
If I combined gimmicks to create the worst pony ever...
Sweet Talkin', Drink N' Wet, BBE ponies that have those weird floral tattoos from the Ponymania set all over them. In addition to their scary doll eyes, they also have the "silly face" gimmick like that one Pinkie and Dash. They're oddly jointed like those articulated G4's they made, and they're also Dance N' Prances-as they spin around, they wet themselves!