I'm going to take the conversation in a slightly different direction. My theory is that no one restriction (or even two or three) is going to change the meta or the amount of tier 1 or 1.5 decks available. We are at a point where we have 3 choices: trim the restricted list or increase its size substantially or change the way we do deckbuilding (IE: more than 75 cards) That is the only way we are going to see any measurable difference on what kinds of decks are played.
The reason for this is our card pool. The card pool has done nothing but grow over the years. As of Dec 2016 there were just over 16500 distinct MtG cards. We get 75 slots to fill a deck. Figure on average 25 mana sources that leaves 50 slots for spells. Depending on the deck we have a handful of spells that are taking 2-4 copes in slots (this is a little sloppy now) that leaves probably 30 or so unique spells in any given deck. If we are to build the best deck possible with 30 spells out of a card pool of 16500 cards, you are going to consistently see the same spells over and over again.
I think we're at a point where consistency in strategies is near absolute. Shops has 4 different spells with sphere effects, waste, strip, ghost quarter and a plethora of artifact creatures that can attack on different axis.
Mentor has so many cantrips that are 0-1 mana that even restricting 1 didn't really impact the game plan. You take away mentor and we have managorger hydra or young pyromancer to replace it. Draw spells? Mentor is a blue shell - it has efficient draw spells out the wazoo.
Even BUG and human decks have a solid consistency to them. Standstill? Get man lands into play (even restricting mishras factor would leave many choices) and just disrupt your opponents game plan completely - has redundancy. Daze and mana leak aren't even considered playable anymore yet a decade ago it popped up in multiple lists.
Oath of druids - look at the diversity in oath builds. From the old meandeck shell to salvagers oath to saheeli.
I said we have 3 options. 1 of them isn't truly an option. We aren't going to start suddenly building 100 or even 80 card decks and the DCI isn't going to change its rules just for us. So this leaves us with 2 realistic options:
Revisit the restricted list and figure out what can come off of it. This would add an influx of new cards that could add new decks to the field. Or it could do nothing
Determine what kind of a metagame you want. And by that I dont mean, "I want a mentor/shops meta." I mean, "I want a meta with at least 5 top tier decks, 5 1.5 tier decks and room for any random number of rogue decks to compete." Then you wield the restricted list like Paul Bunyan's axe in a forest, chopping down cards until you achieve that goal. And you may never achieve that goal.
To me, Vintage is the format where I can play nearly any legal card printed. Option 2, for me, is an affront to the format. If you want to do that then you are probably more in tuned with Legacy.