What is the next pair of jeans I should buy?

Personally i consider any change on the patch as part of the Evo. Shrink, fade, darken, wrinkle, crook, etc just part of the story.
But if I wanted it to stay the same I'd try putting on some Obenaufs LP leather care on it.

...some more:

www.greasepointworkwear.com
www.redcottondenim.com
www.tellason.com

Yea, I've gone from one pair of jeans three hours ago to about four pairs in shopping carts spread across the interwebs.  You have unleashed a monster.  Goddammit.

I wanted to pipe in on this, because there is a little bit of a misunderstanding here. When Gustin QC doesn't bite them, they make fantastic jeans (as long as the fit works for you. You get a flat-felled inseam which a lot of the Japanese brands don't do, loads of chain-stitching, and overall great construction. When the QC bites them, though, it can be bad with popped seams, rivets, and sizing issues. These are things that can happen to all brands, but many make a large run with the plan of throwing out up to 25% that don't meet spec, which creates a number of issues in the Gustin model. All in all, though, any jeans can experience the same issues, and sometimes these issues (such as loose seams where the fabric slipped and the manufacturer turned them right side out and shipped them anyway) will even bite you on expensive pairs. It just comes down to this: the unfortunate truth of the Gustin model thus far has been that the customer takes on the intermediate quality control that would usually be taken on by the retailer, as each pair doesn't get individually inspected before shipment (if this is not true, my apologies, but I've been paying too much attention for a couple years now). If that's not a risk you're willing to take, you're better off going to a retailer and paying their markup because they're offering a service that you place great value on.

There's a lot of discussion on the Oka Standards somewhere in this forum, you may want to do a search and take a look at the commentary. Except that from ickes because he hates his okayamas (it still feels like the "sick fades" comments from Gustin are slightly directed at you, ickes).

Also US made:

Leftfield NYC
Bravestar Salvage
Shockoe Atelier
Norman Porter
Norman Russell
Noble Denim

If it's not going to hit warm/hot water, I'd wax it which should have the least impact on color. If you don't care so much about the color of the leather, but want it to be resistant to color transfer during a soak/wash of the denim, water-proof it with whatever you typically waterproof boots with. However, I find that most of the indigo transfer on patches comes from my belt rather than washing, which waterproofing is going to do nothing to help, and the belt will likely still transfer indigo right onto the leather.

You may drive yourself crazy with all the great options already mentioned by this knowledgeable crew. I would simply get the pair that fit me best.

Only $300 and they're not already in your closet?  =x

I know RailCar and Left Field are in-house production, and I believe Rogue Territory is. If you have option to try all those on, and price isn't much of an issue, I don't think you'll have any trouble unless the fits just don't work. Where Gustin runs lots of fabrics, these guys typically don't, so anything they choose to run with is usually pretty well vetted, Left Field probably being the most fabric-adventurous of those brands. Gustin tends to run the Okayama Standard about once a month these days, I'd say.

So now that I've settled on Japanese denim, what does it mean when denim is from Okayama prefecture? Does that mean there is a single denim mill in that part of Japan, or do all the mills in that area produce denim with similar characteristics? Is there a strong regional difference in Japanese mills/denim, like BBQ here in the US?

Okayama prefecture Is to Japanese cotton/denim As Detroit is to the American auto industry.