Long time player, first time poster.
I've been thinking a lot about Vintage this week with Champs on the horizon. After a long week of experimenting, and contemplating the state of Vintage, I came to some conclusions.
You Need a Reliable Way to Disrupt Your Opponent on Turn 1
I think it's safe to assume that Turn 2 is when a Vintage deck essentially wins the game if unopposed. Even Legacy allows for decks that win the game with consistency on turn 2. But you're not going to win every die roll, so before your opponent enters turn 2 first, you need something disruptive turn 1 on the draw.
Shops is the Only Deck that Can Truly Rely on Thorn of Amethyst
Shops is the only deck that can cast Thorn of Amethyst on turn one consistently. Yes a fully powered deck with Ancient Tombs has let's say 12 ways to Thorn on turn 1. The problem is that a Workshop deck will always do it better because they also get Workshop to add to the consistency. Trading Sphere of Resistance for Thalia, Guardian of Thraben is not equal. Yes you get a creature which can be harder for your opponent to prevent/remove, she's less reliable to cast on turn 1 vs casting a Sphere in Shops.
Force of Will Pushes You Into Keeper and Away From Fish
FOW, MM, and Flusterstorm are real ways to disrupt your opponent on the draw (without playing Shops). FOW is the real deckbuilding constrainer in that line-up (and Flusterstorm on your mana base). You need to play a bare minimum of 16 blue spells to justify FOW, but the more the better. That's not a bad thing per se.
The problem is that you don't need a 20+ creature suite to win the game. There are plenty of 1 card combos or win conditions (Oath of Druids, Monastery Mentor, etc). And when you don't need to dedicate a bunch of slots to threats, you can play more cantrips and disruptive spells. Cursecatcher pushes you to play Merfolk, whereas PreOrdain invites you to play all the most powerful blue spells available.
So if you want to change up the top blue decks, you restrict the cards that allow decks to only need 6-7 slots for things that actually win the game. But I don't think the lack of diversity in Blue is the true issue.
Your Deck Can't Depend on Thoughtseize And Other One Mana Spells
Other colors offer ways to be disruptive, and those other ways can be played Turn 1 on the draw because they cost one mana. The problem is that they cost one mana. Every blue deck plays a significant number of Mental Missteps because they need to keep other blue decks honest. This pushes out decks that might trade counterspells for discard.
Even worse, it's hard to justify building a deck that leans on one mana threats. When blue vs blue have the same counterspell suite, which includes MM, Delver of Secrets or Goblin Welder (or even Death's Shadow) line-up terrible when compared to Mentor. A strategy trying to go under threats like Mentor/Oath are punished by MM. Also MM is a card any deck can play, so even something like Dredge could punish a 1-drop based deck.
Conclusion: Restrict Mental Misstep
Full disclosure, I never thought I would suggest restricting Mental Misstep. I actually love this card and have enjoyed playing with it for years.
That said this restriction would allow for true innovation and metagame shake-up. To put it in perspective, I would be glad to see Mentor restricted. He's a very oppressive creature. But I don't want that to happen (not now at least), because it doesn't change the problems laid out above. If I can only play 1 Mentor, then I just adjust my 6-7 slots for win conditions. Maybe that means I'm a PO combo deck or Oath deck now, but the result is the same: you still can't really play Delver, Welder, or Thoughtseize.
Making room for 1-drops again allows for different strategies, and helps out under represented colors. If anything I'm excited to take on the challenge of building successful decks in a 1 Misstep world. It would truly be a new frontier.