I don't know. I'm going as a wolf in sheep's clothing, but that is a more complex costume. There's the classic fallbacks like vampire and zombie and ghost r witch or whatever, but those are- well, they're fallbacks.
One of the best inspirations is books and movies- maybe a favourite disney villain?
(one of these or summat)This is probably more work than you really want to put into it, but...
Oh, and pirate you can get really creative with. Find a puffy shirt, shorts, and big ole' boots, that's all you need to start, and then you go for details- a sash, earring, cutlass, etc. The more the better. Tear out a button and replace it with another, different one if you can sew. Hair in a bandanna- don't bother with a bought pirate hat, bought costumes SUCK.
et cetera....