I think making some small little detail unique can really make it - if you're in a sea of My Little Pony cosplayers but you took the time to include something like a bunch of candy (for Pinkie Pie) or a particular book (for Twilight Sparkle) then that can really make you shine.
Or if your costume calls for a particular colour trim, but you add just that little bit of sparkle by using a slightly fancier type of trim, that too can really help.
Princess Bubblegum (Adventure Time)
Marceline (Adventure Time)
Suiguintou (Rozen Maiden)
Twilight Sparkle (Princess - My Little Pony)
Rest to be decided.