Struggling to find a good name (I've also pondered dreamwalk and shadowstep), but this seems like a good idea for a creature mechanic. I'm sure someone else thought of it before I did, of course.

One of the desirable traits for a creature mechanic is to not be another evasion ability, though, because blue already has too many of those.

Yeah, I know. :-/

I haven't seen this come up in previous discussions like Blue/Red keyword ability. It reminds me a bit of "sidewalk" from Dr Jones's set Orobis, which said "This creature can be blocked by tapped creatures but not by untapped creatures". That had a rather interesting dynamic. This just looks pretty close to unblockable which isn't so interesting.

The current evergreen keywords fall mostly into two categories: abilities that modify how there creature functions in combat, and abilities that protect the creature. The main exception that I can think of is fight, one of the evergreen keyword actions. None of the abilities provide boosts to power and toughness. It's a difficult set of restrictions, because the majority of space left seems to be in different tweaks on evasion... Which is what we don't need.