Interesting ideas ...
There is a jetpack in Mekanism by the way - the only problem is it doesn't work in space just yet, but that shouldn't be too hard to fix.
My intentions are:
1) Get the current 'vanilla' movement in space better, that mostly means jumping and landing.
2) As discussed here:
https://github.com/micdoodle8/Galacticraft/issues/630 we might look at something where the player model can be tilted (away from the vertical) for more freedom of movement in freefall - I'm not sure about going all the way around and upside-down though, that could be too disorientating for some players.
3) Get the Mekanism jetpack working in space
4) Based on experience with that, see if we need our own jetpack.
Since some movement now requires proximity to blocks to "push off of", perhaps when a tether is attached you can move easily without delay directly in the direction that the tether is attached to simulate you pulling the tether tight and therefore pulling yourself back toward the anchor point. This should only work TOWARD the anchor and in no other direction because you can't use a flexible rope to push away or sideways. Perhaps you could simply press SPACE and it would automatically pull you in no matter which way you are facing?
I like this, but me + Micdoodle8 will need to think in some detail about whether it's going to be possible to implement. The main difficulty with the tether is you're going to need to see it in the game, and it's going to be a rope-like object filling many blocks - that's kinda hard to draw in Minecraft, though I guess there are fishing lines and tripwires so it's not impossible.