There seems to be a bit of confusion about how we handle kernels at DO. Hopefully I can clear things up a bit. On newer distro versions (e.g. Ubuntu >= 15.04, Debian 8, Fedora, CoreOS, FreeBSD) we no longer use "external" kernels. You are free to compile and use custom kernels. We're happy with what we've seen, and with Ubuntu 16.04 around the corner our default distribution will have support for this as well. After that point, we'll be backporting the change to older releases.
For Droplets still running with external kernels, we import new ones on a regular basis as they are released. If you happen to need one that hasn't been imported yet, just open a support ticket and the team will do so.
It's like on any Linux or BSD machine. You can build and install the kernel normally, and it just works.
Only some of the older images still use the old method of selecting the kernel from the Control Panel. The rest that Andrew mentioned use the bootloader and kernel from the droplet's image itself.
Are you planning to offer storage options? I find it's ridiculous to have to upgrade to a higher CPU/RAM plan (or add a new node) just because we're running out of disk space. We don't need detachable volumes or anything fancy, just the ability to increase the amount of disk storage.
Storage is also an area we're working on heavily right now, and we should have some good news soon. We definitely recognize that there's a lot of demand for more storage without the need for the corresponding upgrade in compute power. Check out the update from our product team on this UserVoice request, and vote/subscribe to get updates:
Product manager for Storage at DO here. I generally avoid comment on these kind of things but given the public statements already out there it is safe to say we are launching storage reasonably soon. If you are truly interested in participating in the beta program, email me at tfrietas@digitalocean.com
For Droplets still running with external kernels, we import new ones on a regular basis as they are released. If you happen to need one that hasn't been imported yet, just open a support ticket and the team will do so.