What is the reasoning behind leaving PO unrestricted while Gush remains restricted? Gush only generates a good deal of mana with Fastbond. Without Fastbond, Gush is a draw 2 that can return two lands to your hand, and maybe generate 1 mana in addition to mana from tapping those lands. PO draws a lot of cards and makes a lot of mana. Sure, it needs artifacts in order to work at all, but usually none of the artifacts are dead draws on their own, and most combo decks want them regardless of whether they use PO or not.
I'm not advocating that PO be restricted, but it seems to me that the more interesting blue cards are restricted, at least in part, for the sake of PO staying unrestricted. I wish Gush were unrestricted for good. Restrict the problem cards like Monastery Mentor, not the related cards they happen to use, like Gush.
I'd personally like to see Brainstorm unrestricted. We have Mental Missteps, Mindbreak Traps, Pyroblasts, and many other popular 0-1 drop counterspells that would keep them in check, not to mention sphere effects in creature and artifact forms that can easily be cast T1.
I don't think Brainstorm would be an automatic 4-of in all blue decks; usually, there is a ceiling for how good Brainstorm can be in each turn. Unlike Ancestral, if you don't have a means to shuffle, Brainstorm is a very temporary solution to whatever bad hand you might have. Having multiples of Brainstorm often doesn't generate too much advantage for the same reasons. It's telling that Portent, a synergistic card also released in Ice Age, has you draw during the next turn, instead of the same turn it was cast, acting as a kind of check to Brainstorm. I doubt the interactions between Brainstorm and Portent were unintended by the creators. With Ponder restricted, I doubt Brainstorm would be at a power level worthy of restriction in Vintage.
Brainstorm is also one of the more nuanced cards in the game that would add strategic depth to the format as a whole without increasing the power level of certain decks too much. For example, having a way to consistently hide cards from your hand would lead to more decisions informed by more considerations for both players than not having a way to do so, while not giving an overly 'unfair' advantage (which I would loosely define as literal card advantage for little resource cost) to the player with Brainstorm.
I also personally would like to see more grindy control decks that do more than cast Preordains and a couple of tutors for library manipulation. Decks with draw engines are fun, but there are too few oppressive or sneaky blue decks that are more in the style of 'draw, go.' Almost every deck right now is just so busy to find its combo piece or lock piece. I believe unrestricting Brainstorm is one of the ways to help improve blue deck diversity.