I like to redress dolls and here is what I've found:
Some Hairdorable Hairmazing stuff fits the dolls. In fact, it all pretty much fits but MH have longer torsos so the shirts may be more cropped. The Hairmazing shoes fit the Big Sister size perfectly, but so long as you don't expect the dolls wearing them to balance on their own,the shoes also work for regular MH dolls too . Boxy Girls, and Snapstars both have clothes that also mostly fit (again, it was just the different torso size, so avoid one peices, or alter them to be two pieces). Boxy girls shoes will fit the little sisters, again, so long as you don't expect them to stand balanced on their own.
Wild Heart Crew stuff fits the boys pretty well and was a good aesthetic match for Monster High. Wild Heart Crew shoes fits the Little Sisters almost perfectly.
Creatable world stuff works well for the dolls. Though they are more solid color/basic patterned ote,s than Monster High's monster-theme, they can mix and match with MH stuff really well. I especially like CW stuff for the boys. Shirts fit okay, jackets fit well but may be cropped, depending. Bottoms can fit but will need some adjusting. That's why sticker-backed velcro was invented though. Pants will probably require some adjustment, but may work.
OMG doll stuff can be hit or miss as far as fit goes. As a general rule of thumb, I've found separates work better than one pieces items. The tops can fit the Big Sisters almost perfectly (if cropped), and the little sisters also can fit these items, with a little overlap of cloth in the back. Jackets will fit little sisters, and regular MH dolls but will look cropped. Stretchy pants will fit Big Sisters but be capri length, I've found. I think some of their prints and designs can pair really well with different ghouls' themes.
Hope this helps!