Display stands.
I’d love to have a bunch of simple, yet elegant stands (plain wood, perhaps). Something that you can put on a shelf and display individual yoyos.
You could make them so that they consist of just 2-3 pieces that fit together for the buyer to assemble (easy shipping!).
I would like a delrin multitool with a string cutter, bearing remover and pick. The bearing tool could be metal, but a pick, like the MFD multitool, but in delrin to not scratch the ano would be greeat for picking knots.
I would be happy for just a delrin tool with string cutter and knot pick that would go on a key chain. This is what I need 99% of the time anyway.
It should be designed so it doesn’t poke a hole in your pocket, and doesn’t scratch your throw.
I like it! I’m gonna have to look into this now!
It’s sort of been done before, humanizing them, 
but why not make other small accessories for us weirdos.
http://farm8.staticflickr.com/7350/9604472259_df51ab82df_z.jpg
MonkeyFistPlaysKeys2 by The TotalArtist, on Flickr
http://farm8.staticflickr.com/7402/9619271095_545535d0c0_z.jpg
MCACandDJCHIEF by The TotalArtist, on Flickr
There are a ton of ideas, but I’m not so sure anything is truly “missing.” There is enough room for people to make all kinds of yo-yo related things, so I think you can have your pick. Apparel related to yo-yo culture would be great…aside from companies that manufacture yo-yos and yo-yo accessories. Just a company that does clothing just because, not to promote other products, but to spread knowledge of yo-yo.
Yo-yo stands would be popular. Everyone has an office, night stand, coffee table, mantle or something where they need nice stands to put their yo-yos on. I think people have done well with it, but have not sold them consistently with the right look at the right price.
I’d love to have a good knot picker that will not scratch the surface of the yo-yo. Not many people are doing custom ano, so I think there is room to enter that market as well. :-\
There is also a market for upscale hard yo-yo cases. No one is producing them on a regular basis, and a lot of people have to make their own.
Yo-yo holders are easy, just come up with something that has an element no one else is using, and create a market for yourself.
Ooh… Some sort of cover you pull over your yoyo to protect it from dings… Stretchy wouldn’t work, but maybe cotton that you shrink onto the yoyo? That would be awesome.
maybe it could be the material they use for book covers except with foam padded around the inside so it doesn’t ding!
I think people have tried stretch covers before, but they fly off while spinning.
TA had a brilliant idea when she said upscale hard cases. Everyone has to make their own. I would buy one.
I think if you could figure out how to make a good (but not too expensive) multitool with a bearing tool, string cutter, and knot pick you could make a profit. As previously stated there is not much competition.