RE: marksman & autos. To make marksmen more distinct, maybe crit could be reworked into something that gave different things to different marksmen? Each of their passives or unique on-hit effects could scale with a new "crit stat". Caitlyns passive for example, or Varus' Blight.

Yeah, we talk about this idea a lot. It's more interesting when it is something that players can really leverage, such as "You always crit a rooted target and do X more damage" rather than just making a random stat even more random, like "Your crit chance increases when you do X and decreases when you do Y."
(In a strict mathematical sense, increasing control ends up decreasing the randomness at the expense of increasing the spikiness, but the end result can still be from another player's perspective that you never really know if crit is a big danger or not.)