I would love to switch to this. But there's just one problem. The 6 slot limitation. Most other chainloaders allow somewhere between 10 and 12 hotkeys, I only use 9 to avoid blocking 3 Luma features I find extremely useful (L, select, and A). Even so, going from 9 to 5 hotkeys, well, that's practically giving up the entire d-pad. Maybe I don't NEED all of those hotkeys. But it's what you're used to. If I press one of my d-pad hotkeys I've been using for years, and it does nothing, well, you get the idea. My muscle memory would just drive me nuts (and most likely ultimately force me to switch back to my current chainloader).
I currently use this to add 5 extra hotkeys to my primary chainloader (which renders the CBM9 extras menu a holdover). So it would be nice if you could go to at least 15. Though 24 would be even better. I will be adding more script runners to my AIO in the future (a "Crap Cleaner 3DS" and a third party script collection are already planned), so some extra headroom would be much appreciated.
Maybe you could add a slider or page-over feature for said extra slots. You could even make the maximum amount adjustable. For example, I could set mine to 24 or 30, and others could set it to 6 or 12 if they find having that many too cluttered. It's not like you're hurting for possible hotkeys with the software already accepting multi-button combinations. Which just makes this superficial limitation that much more frustrating.
Considering the primary function of fastboot3DS is to be a chainloader that runs from the firm partitions, I'm just not seeing the point. You'll notice that being able to access the bonus drive in GM9 doesn't make it show up in anything you launch from GM9's payloads menu. Adding support for it to this chainloader would not cause it to show up in the CFWs and apps it launches. Granted, this has a NAND backup/restore tool. However, you would probably need compression to fit a NAND image onto the bonus drive. I'll be genuinely surprised if GM9 (and therefore my NAND Manager SSR) doesn't add that feature first. If you want to install games to it, Luma (or some other CFW) would have to add bonus drive support directly.