I feel range on marksmen is already strong enough as is, as it's allowed certain marksmen to turn into lane bullies and attack melee opponents with little fear of retribution. Perhaps range growth could be a thing, as it's already been added on Gnar to balance his laning phase out, but I also feel changing ranges tends to feel messy and unclear. I feel the main issue with marksman survivability right now is simply that marksmen get hard-focused by the enemy team, so of course they tend to die quickly when that happens. However, Riot's answer to this has apparently been to literally make marksmen so survivable that they can still not die even when being the target of literally every player on the enemy team, which is pretty much exactly the opposite of how I think the problem should be handled. I think this is but one more reason for why marksmen shouldn't be the only valuable carries in the game, and bringing some mages, fighters, assassins, even tanks or supports to similar levels of priority would allow that pressure to be relieved, especially if not all marksmen end up being carries by default.
I also agree, I feel Riot's driving themselves into a corner by overloading their kits. Just look at Evelynn's VGU: I like her new kit, but she now has sustain, hard CC (and soft CC too), two gapclosers and some utility on the side. In order for an assassin to compete with her, they're going to need a similar amount of features or just tons more raw power to compensate, which risks turning them into another Ekko or pre-nerf Zed. Similarly, setting the bar for tanks at 3+ forms of AoE hard CC, plus spammable PBAoE nukes, does not exactly make for a very diverse roster, and it's this kind of issue that I think has severely harmed Galio, Maokai and Sejuani with their own updates.
While I feel class definitions are a little softer overall than they're made out to be (it should be possible, even recommended, for a champion to go outside the typical bounds of their class, so long as they make proper tradeoffs for it), I also agree that class definitions are a little sketchy right now. Marksmen in particular are supposed to be squishy and DPS-focused, yet are among the best drain tanks in the game, purely because of how strong life steal is, and on top of that are also pretty bursty. Mages and assassins are meant to have high downtimes, but that has become increasingly difficult to implement in an age where items are saturated with CDR. Tanks and fighters I feel are the classes with the most overlap, as they're both supposed to be tanky and usually melee, yet only one class is meant to specialize in CC and the other in damage: during Seasons 2 through 4, fighters were considered better tanks because they had tons of damage in addition to decent amounts of CC, but right now many tanks are essentially better fighters, as they have better durability, better CC, often comparable mobility and quite a lot of damage. Designing tanks to have good base damage on their abilities, plus mobility and crowd control, has effectively turned them into more loaded and reliable divers, which is why I feel the Tank Update actually increased the overlap between tanks and fighters, instead of better separating the two classes.
This Preseason may not be the time Riot chooses to overhaul the game's underlying mechanics beyond runes and masteries (which is already a massive task), but I do agree with you, I hope at some point they look back at some of their design decisions and start pruning unnecessary mechanics off of champion kits, and perhaps even items. My personal philosophy here is that a champion should not have a feature on their kit unless it's meant to be an essential contribution they're supposed to make, and I feel there are a ton of effects that don't obey this at all, and were simply there to make the champion feel better, or just to add an extra balance lever in an environment where everyone already has excessive features added to them for that same reason. If the game is so hard to balance that champions need CC or utility added onto their kit to not snowball too hard either way, that should be held as a definite sign that the game itself needs to be balanced around being less snowbally, and merely papering over that flaw has caused a lot of feature creep, and eventually brought us back to a place where even the most overloaded champion can be too snowbally (some of them are even among the snowballiest champions around).