The SDK was meant to be something like an engine on top of the current one of sort to speak, with a lot of new features for both mappers and modders to use, like particles, static meshes, new renderers, post-processing, etc, etc..., all using native C++ code.
However that project is at the very least on hold, although it's most likely completely canceled, and has been so for several years already. I cannot say "canceled" for sure since Shadow
(the developer) never confirmed so, but we have to assume the worst given that several years have passed since the last update, so I doubt anything will come out from it at all.
From there, there are some active projects going on, such as XC_Engine from Higor
, which aims mostly to patch the current engine, and at the same time add some new neat features as well, such as converting brushes directly to meshes within the editor for example.
He has other XC_* projects as well, and he has a section for all of them which you may check here: viewforum.php?f=62
Although not for mapping nor modding, anth
is also continuing the development of his anti-cheat, ACE, which should help on the game longevity and fairness as well: viewforum.php?f=65
I also intend to do some stuff again in the future myself, be in the form of both mods and tools, but right now I am hard at work in the necessary foundation for all of it, and have been since the start of January, for which you can find more details here: viewtopic.php?f=12&t=12684
(it's a very long read, but it has a TL/DR which summarizes all of it)
But I have no idea how long it will take me, but for what it is worth, I have been making progress (not as quick as I would like, but been making it nonetheless).
Other than that, some are still developing their own things, such as new maps and mods, there has been one or another tool released, things like grid texture packs to greatly help the mapping process for example, and other things of a similar kind, but nothing more beyond that (unless I am missing something).