A service that I think would be interesting, and probably not terribly profitable, would be made-to-order clothing. If, say, you want a polo shirt, you might have a choice of two different cuts, a few different basic styles per cut, and a few color combinations per style. The idea here is that you'd have hundreds of possible color combinations, as the buyer could choose all of the colors involved, and you could apply your colors to any of the styles available. You'd never have to worry about seeing someone else wearing an identical item of clothing again.
There are tons of problems with this approach. One is that this would almost certainly happen over the internet, and I'd imagine that the average sort of person who buys clothes off of the internet doesn't care enough about how the item looks to pay for a premium service like this, and to generalize even further, maybe they don't even have a good eye for color. (All speculation.) The process would likely be expensive, and the items wouldn't be returnable, so you'd have to make sure that things were sized properly beforehand. How do you do this? I know the sizing of a few brands, which allows me to buy their clothes online instead of in stores, but in this situation, you'd have to know that a custom XXLT was exactly like a regular Gap XXLT. Now you've got brand names in the mix, pushing prices up higher. On top of that, I'm pretty sure this just isn't the way that textiles are made. There's probably no process anywhere to produce shirts in custom color combinations like this.
I'm sure some clever clothing manufacturing expert could think of ways to trim down costs, but these things would still be expensive. Still, I'd love the opportunity to someday go to a website, pick out a pattern and style, choose my favorite color combinations, and get a custom shirt a couple weeks later. I wonder how long it will be before this idea is feasible... because I just can't imagine it's even close right now.