I'm sort of nervous about giving examples without asking people's permission to use their photos. But... I'll just source them as best I can ^-^;
I've done not-so-serious/well done doll modification before with Bratz dolls, using sharpies to give them tattoos and darker makeup. Some of them came out pretty good, but I plan to take off my previous work on most of them and do it a little better.
The only Monster High doll I currently own is Draculaura, the first/original one, and I've decided to customize her. So far I've just taken off the original paint and taken her hair out of pigtails/washed the weird sticky "hair spray" out of it. I'm not sure what kind of character I'll make her into yet, but I shall post her when I'm done.
One thing I really want to do but I'm not sure whether or not is possible is to dye her pink streaks black. What I'd REALLY like is for them to be a much more pastel pink, but lightening synthetic hair is impossible, from my research.
My Draculaura, pre-modification
I hope to get a few more dolls, the ones I'd like most are Frankie Stein, Ghoulia Yelps, and Spectra Vondergeist (I love that name :P oh lovely made up German surnames ♥).
I think doll modification will be a fun thing for me to get into, it's drawing people, but on a 3D surface ^-^ I'll also get to sew the dolls clothing, which will be sort of difficult (they're so tiny and oddly-shaped!) but interesting and good practice.