The interpretations of these hits will vary across the board.
Exactly. I have no problem with an ejection for a clear, flagrant hit. Where do they draw the line though.
That would be my concern, too. Maybe a more-or-less incidental but hard helmet to helmet, which seems to get called quite a bit, should get just the PF call, and the ejection should be reserved for when it's clearly deliberate. Same deal on crossing pattern collisions -- it seems to me there's a difference between a hard hit that could get flagged because the hitter could have avoided it (at the risk of a big play against him) and a guy clearly nailing somebody who doesn't have a prayer of making a catch (or tackle). I'd be afraid that any new rule would lump those things together; better to let the refs decide and just encourage them to be tougher on the truly bad plays.
Coaches and players will go right up to the line (and a little beyond once in a while) if the line is a bright one. Any rule that codifies the 'ejection' line will just create a new standard for play, a standard nastier than what's happening now. Make them know that that line is a little vague, and it has a better chance of actually keeping the unneeded viciousness down.