I'd prefer nextCSS with all the spec being based on what is actually going to make in into CSS. Otherwise it's the same downsides as preprocessing languages with the added bonus of not being able to easily tell what lines are native to css or not.
And to clarify, yep Rucksack certainly isn't to-spec (present or future). It's a collection of handy features and shortcuts, like the compass/nib/bourbon of PostCSS. As timdorr points out, you could absolutely use it with cssnext (we do exactly that over at Simpla), you'd probably just want to disable autoprefixing, since cssnext does that for you.
But adding new spec that looks like the CSS language is not transparent, it's ambiguous. It's not a knock perse. And like you say it does not lock you in, as you can rely on the output. But I wouldn't want to use it for projects that are distributed to other users, because now they have the burden of learning and distinguishing between constructs coming from rucksack and normal css spec.
If so, that is also built on PostCSS and could be combined with this pack. Rucksack is just a packaging of other PostCSS plugins: https://github.com/simplaio/rucksack/blob/master/index.js#L6...
But feel free to chuck an issue up on github, this behaviour probably needs to be accounted for sooner or later.
