Just those two should do it. With enough population, Matchmaking would act as a split queue (ie, if its set to sort premades or pug,) while still offering the benefit of filling for partial premades and backfilling for all.
Other than that, you are right. Although I would love for more people to stand up and take responsibility to become more competitive, Cross realm pvp and matchmaking are needed to be less discourage for those whom aren't quite up to par and/or are still learning.
Back filling in it of itself is rather dysfunctional the way it is now but I won't argue that topic. Back filling a premade game with pugs should only be an option after the game has initially been formed, ideally matching Premade Combination A Vs Premade Combination B, using pugs to fill in the limited, talking anywhere from 1-4 spots maximum, for either side, again leaving this as a last resort if other grouped players in any combination cannot be found to fill in the gaps. Back filling these games when in progress should be done sparsely and only when the game is not an obvious victim of the "Jump Ship" syndrome; these type of games need not be backfilled and left to the Wz shutdown sequence to run it's course. Nothing worse than wasting a player's time when they are thrown into a game where 2 or more players decided to leave because it appeared to be a losing match. A steeper but reasonable penalty also needs to be implemented to discourage "jumping ship." Better yet an incentive to play the Wz to the end, despite it being a loss but all the more fruitful to push for a win or as close to a win as possible. A linear system as we have now, heavily encourages winning but offers very little to complete the Wz otherwise and has no penalty for abusing the leave Warzone option.