What I'd like to see is something that avoids a TCAS RA in as many cases of potential conflict as possible, because a TCAS RA relies on both pilots reacting instantly - and correctly - to avoid what happened in Brazil. Unfortunately, the Lake Constance collision proved that despite the best will in the world and God knows how much training by airlines, you can't rely on the other pilot to do the right thing. 100m offset won't stop an RA being generated (nor would .5NM, but I'd feel happier with that if it didn't upset ATC too much).
You're never going to come up with a system that covers all eventualities. With a crossing conflict, both aircraft have to reach that one point in the sky at the very wrong moment. With reciprocal tracks, the two aircraft could be occupying the exact same line along the ground for hours.
Surely Andu's idea of the offset being enabled only when in LNAV has some merit. Failing that, just give pilots permission to use offset in the cruise in all but certain designated areas, (as they already do over the North Atlantic). Offhand, I can't think of any area in the cruise that would need to be excluded, (but I'm sure someone will come up with one or two).