Here is what I suggest and see if you think this might work. Find out who is in charge of coordinating the spirit days, then approach them with the idea that you would like to change what the days will represent. When they ask you what you are considering doing, tell them you will have sheets that students can sign with ideas in a specified location in the school. If you could do it online, it would be easier, but then you don't know who is doing the entries. Then when you have a ideas, you will make a survey for all the kids to vote on.
If you feel this way, then I am certain others do and you will have made a difference!
Dress like your favorite teacher??????????? UGHHHHHHHHHHHHH.
Wear the colours of your favorite sports team???????????????? UCKKKKKKKKKKKKK... I don't have a favorite sports team. Professional sports are NOT something I care about.
Of course, you could always start a petition which addresses the changing of what you do on spirit day and start having kids sign it. Then when you get enough signatures, you could present it to the person in charge.
I think that's actually what we've done in the past, and that's the best people can come up with .__. The thing is, I always end up forgetting about Spirit Days anyway so I've never made much of a fuss about getting cooler themes. But I do know some people who have been talking about doing just what you said.
And then there are the cool people who come to school dressed as pirates anyway and find some sort of vauge connection to the theme if anybody questions them. Like, "This is what my favorite sports team would dress like if the team was entirely composed of pirates."