I kinda like what you're saying OP.
I mean, right now trying to get any character is kind of a pain and setting out to get one specific item is really a game of chance. I mean, I've been wanting the Cabal Vanguard since I got the game recently and I have seen neither hide nor tail of it. Your option doesn't really make it any more or less probable in the beginning but in the long run makes the grind for characters or weapons a little bit less... Grind-y.
But I guess that depends on what you're proposing. If you say "Untill every weapon is level X in common don't move onto uncommon" that would be... Mightily tedious. It kinda makes certain that you are going to be banging your head against the wall in the beginning. Either they would need to diversify the arsenal of items in each category (the uncommon area characters and to a lesser extent weapons is a graveyard compared to rare). If you're saying that we get the max level of the weapon when we get the card the first time then there might be a bit less of a grind. Again, it would depend on if the number of items in each tier. We have twelve characters in the basix set, right? That means that if you average 2 characters in the beginning from a reserves pack you are going to have to spend a guaranteed half-million credits to move onto the uncommon chars. Getting to Ultra-rare? That would not be pretty. Weapons would similarly be a bit of a hassle.
That's just my thought on it though. I've thought about what others have sometimes proposed, the in-game store to buy things with credits specifically but as the second (or was it third?) poster said there would be people with Harriers, Jugs and Warlords on higher difficulty levels that are going to get themselves knocked flat.
If the store item availablity was tied to your N7 rating though... That might smell a bit too much like "The fish we cannot name" for some people's liking.