MARK IT ONCE on the green is the one I think that should be implemented. It will save 30 minutes on an amateur round of golf.
After your first putt you are not allowed to touch the ball, if its in the direct line of the other player he can ask you to putt out.
No penalty for striking an opponents ball or flagstick.
Saves the time of aligning the ball with hole a second time. Saves the time of remarking.
Much less compaction around the hole.
I like the new two stroke penalty option, will stop a lot of NO RETURNS as well.