I think this is not a great idea. Imagine how complicated the restriction decisions would have been on the Wizards' part. And unrestrictions would be even worse. For example, we can easily imagine a card that most people would think 4 of is too many. So what about 3? 2? There would certainly be people saying 2 is best, others would say 3. How do you make an argument in this discussion? The math is quite complicated and the difference is not as obvious as between 1 and 4, so I bet most of the arguments would be based on a feeling or be just opinions on the matter. This would mean somebody has to do some very serious research (in the case of vintage probably infeasible due to small sample size) or there would be even more disparity in the community (regarding restrictions) than it already is today (not going to mention LSG here).
So basically, as The Atog Lord says, for the sake of our own sanity we shouldn't complicate the existing system too much.
PS: It is kinda like predicting weather (among other things). You can most of the time, with some effort, guess if next day will be hot or cold, but nailing the precise temperature is very hard. The finer-grained you want your decision to be, the more likely you are going to make a mistake. And if you don't have enough data, you are just guessing.
PS2: This is my first post on TMD, so: hello everyone! I was lurking for quite some time, but this topic caught my attention enough to warrant a response.
@snowydude Direct comparison of draw spells is not as easy as it is with burn (I mean burn, not removal, because you mentioned Chain Lightning). All you are concerned with burn is efficiency (and sometimes uncounterability). To compare draw spells you have to do much more complex math. In fact, it is complex enough that (from what I observed) there is no general consensus regarding the best mix of cantrips and draw spells.
I don't think Gush competes for a slot occupied by Preordain or Ponder. Also, in general I would play 4 Gitaxian Probe before 4 Preordain in a "Gush deck" and a lot of people do the same. Many lists play only one delve spell too (or even none). It is very deck dependent. And I guess playstyle / pilot dependent as well, given that people don't agree on a single build.
PS: I'm not sure if it's correct (of course), but I play 3 Gushes, 4 Probes and 0 Preordains in my Esper Thing in the Ice deck