I’m not in any way official, but I can venture a few very good guesses to a number of these questions.
a) A token with only 3 and 4 star characters is quite a bit more valuable than a normal heroic token. It would have to be distributed at a much lower rate than normal tokens.
b) Probably, as there is no other good defining way to distribute them. I suspect that they used the off season as a test to see how people liked them, and will develop things further based on that.
The presumption for LTs is that the latest three 5s are the “premium content” while the remainder can be treated just like any other tier. Given the rarity of the 5s, I don’t think that’s quite accurate, but at least Bonus Heroes gives us a bit more ability to chose what we get. This expectation, however, means that the two token types should be identical other than the 5s it draws from. The reason for vaulting doesn’t change just because players are willing to pay the premium (an extra 5 CP) for the latest pack or not. They want to limit the 4 pool to make it easier to complete characters, and the most recent 4*s is the obvious way to limit it.
Like the 4* vaults, they are probably experimenting with various iterations of the token.
I believe it was 4-6 months ago there was a Q&A on one of the chat channels - it was stated that there were plans for a 6* tier, but these plans were scrapped. We may get a 6* tier at some point, but I wouldn’t expect it for some time. As far as the 4* tier goes, they clearly intend to introduce more ways of getting 4s rather than seriously tinker with the reward structure (e.g. give out more 4 in progression.) This may change with more Security levels.
Vaulting as it exists did not represent a huge coding change - changing what characters are available in tokens is something they can do for every pack. Making it customizable would represent a very large coding change, and such things take both time and investment on the part of the developers. I will point out that the Bonus Heroes system (like it or not) almost certainly represented quite a bit of investment on the developers’ part.
Bear in mind that with any system some things are easy (changing the cost of tokens, the characters, and their percentages,) some things are a fair bit of work (new PVEs, I expect, probably because there’s an approval process from Marvel,) others represent major changes (Bonus Heroes,) and some things are well-nigh impossible without changing the underlying code of the game (colorless covers.)
Anyway, that’s what I’ve got, Brigby or someone else may add to that, but bear in mind they aren’t really allowed to speculate, so you may not get much.
