I'd be okay with the grim reaper at any age. I consider that more spooky than gory or creepy. The only reason I set limits on gore or scare was for the other kids I knew would be around on Halloween night. We live in the middle of town and I've seen some pretty traumatized toddlers.
But, if she just wants to avoid a girly outfit, I'll throw two ideas we've used in the ring.
Old lady or old man (please seniors do not take offense). We borrowed a walker, took the lenses out of some big glasses, had a white wig, found some older style clothes from goodwill and my mom's house. A floral scarf to hold the wig in place. Clunky Jewelry and Strong flowery perfume (we used room deodorizer, strong stuff, lol).
She and her cousin did this together and pretended to be sisters, they talked loudly, and she carried some childhood pictures and showed off her 'grandbabies' to any one who would look. They joked about which types of candy would be bad for their dentures.
DD had a bag attached to the walker for people to put the treats in. People LOVED it. We have never talked to so many strangers. Even the families passing us on the sidewalk were getting into it.
By far the most fun we ever had trick or treating.
Tourist. Very easy. Just mimic the worst outfits you've ever seen at Disney.

Don't forget the map (or grab some tourist pamphlets from a local rest stop), fanny pack, giant camera, and souvenir cup. Especially fun if you have a group. Pretend you are lost. Ask directions. Inquire about the best place to eat. Etc. This also got some good responses.
Good Luck!