As the world of mountain bikes gets more and more diverse, things get... more interesting for a framebuilder. I've got a big drawer of obsolete tools and parts that are only 5 years old (need a jig to install an ISCG mount or direct mount front derailleur, anyone? 150x12 through axle dummy? 1 1/8" head tube stock? Didn't think so...) and I'm constantly getting presented with odd new problems to solve.
But hey, interesting times. And all the options out there are just simply awesome for riders. The bad old days of 71/73 and 2.1" tires are long gone (though you can still have that if you want it!)
Because I build a decent number of plus size (ie, ~3" tire) bikes lately, and I have a lot of picky customers who don't want just a big tire slammed into really long chainstays to make it all work, here's the plus-bike wish list, as brought to you by someone who builds them and rides them too.
First off, if you're someone who fits nicely on a 29er, consider that 29+ is really 30.5" diameter, give or take a few mm. If you're close to toe overlap on your 29er, or you really feel like the bike is close to too long/unwieldy - then you should do 27.5+ (about 29" diameter), not 29+. Likewise if you're pretty small, 26+ exists and is a great option (it's basically the same diameter as 27.5, but wider tires). Don't just decide you want fatter tires and automatically upsize your wheels at the same time - that might not be appropriate for you and what you want to do.
The most important thing to do for these bikes (regardless of wheel diameter): wider than normal chainline. Let's assume you won't run a front derailleur. That's a pretty safe assumption these days with 1x giving up to 4.4x1 overdrive - enough for anything that will mostly be ridden on a trail for 95% of riders.
The battle, as always, is fitting the chainstays, tire, and chainring all into the same small area near the BB shell. A quick look at any mountain bike will show you why this is trouble. Every extra millimeter we can squeeze out is worth it's weight in gold, so throw the usual ~50mm chainline out the window.
How? Lots of ways, in order of max tire clearance/short chainstay awesomeness:
1. Boost (ie 148mm hub spacing, ~52mm chainline)! It only gets us 2mm of extra space, but that's not a bad thing. If you do nothing else, you can do a boost rear end/crank.
2. Run your chainring on the outboard position of a triple crank (~56mm chainline) and offset rear dropouts/hub to match. Alternately run a direct-mount chainring flipped over (ala Raceface Cinch) or BB30 ring on GXP crank (SRAM). This helps a ton and also has the sidebenefit of a mostly dishless rear wheel.
3. Run an 83mm BB (~55mm chainline) and matching cranks. There are lots of great options out there now though finding 175mm cranks in XC-ish configuration can be challenging. Offset rear wheel to match. Or build around a 150/157mm through axle rear hub.
4. Run an 83mm BB and offset ring (~60mm chainline). Offset rear end 10mm or so to match or run a 170/177mm fatbike rear end (ala Kevin's low-Q fatbike). My favorite option, this actually gives enough room to run a 4" fatbike tire if you so desire and plenty of space for short, short chainstays with loads of tire clearance.
5. Run a 100mm BB and 170mm rear end. If you're going for a fatbike with plus-tire/summer setup, this is the next step up. Some folks won't like the Q factor, though.
I get a lot of pushback on all of these ideas from people because it seems some folks are really stuck in the "bike should be symmetrical" mindset. Here's the thing: your bike is already not symmetrical. You have a drivetrain on one side, brakes on the other (assuming disc brakes). Your wheels have the spokes coming out of the flanges at dramatically different angles. In some cases your rims are drilled offset to one side...etc. My goal is to make the bike that rides the best for you - and in most cases, having the rear end symmetrical is going to detract from doing that with many of the plus sized tires that are out there. Yes, you'll have to have a dedicated rear wheel for your plus bike that you won't be able to swap around with your other bikes without redishing it. C'est la vie. Wheels are cheap (relatively speaking).
Q factor is another question. If you really want narrow Q, options get limited. But fatbikes have proven (for *most* people) that Q factor isn't as crucial as you might think. Adding 5mm to each side (ie using an 83mm shell instead of 73mm) isn't even noticeable to most riders. You might be the exception, of course, but there's no reason to think you need to worry about it unless you have a preexisting knee problem and you know the Q factor is going to be trouble.
So bottom line: if you want a semi-fat/plus bike, consider letting me go a little nuts to make it awesome. It's going to be weird, yes. But much more fun than a cookie-cutter setup with 45cm chainstays (unless 45cm chainstays is what you need, of course!)