I do want it to be toggle for coast. Thing is, coast simply ignores translational input. That's why rotation still works. It's to allow you to pick a direction, and then 'coast' so that you can turn your ship around, and fire at people behind you.
Now, what do you mean by 'more inertial'? Currently, we're doing everything with target velocities. We are using an accurate physics simulation, however, we're letting the game figure out what thrusts we need, given a desired velocity on a given axis. It's the most intuitive, and flyable thing we've been able to come up with.
Are you looking for something where once you srart movement, it keeps going? That would be interesting... setting the target velocity to the current one. then, toggle it back, and it acts like it currently does. Hmm... I think we could easily do this.
I guess the only real question is... what advantages do you have with this?