I am on record that I think the game would be improved by adopting interchanges as used in RFU National 1 and below.
http://www.englandrugby.com/mm/Docum...17_English.pdf
But I would reduce the bench in Internationals and Premeirship to 6 which would be a maximum of 10 interchanges.
National league has a 5 man bench, so that means one prop on the bench and therefore one of the three props has to be able to play both sides.
With 6 you can have two props, a hooker, another forward and two backs. Which is sufficient for 99% of games.
I believe it is 4 subs at level 4 and 3 below that.