The Conundrum of ADCs and the Botlane
Recently, there has been a lot of discussion about ADCs and their effect on the game. Allow me to offer another, slightly different perspective.
First, why does virtually every game have at least two ADCs in the gold-gaining role in the bot lane?
Simple answer: Because this kind of champions is less negatively affected by having access to less XP than the other classes in the game. That boils down to two factors:
For one, range is its own form of survivability. Having an attack range themselves allows champions to stay out of the enemy focus, meaning that they don't need as much stat-based durability to survive, as they are far from guaranteed to take damage - can't hurt what you can't hit, can you? Contrast melee champions, who by necessity move into range of the enemy focus, meaning they are virtually guaranteed to take the enemy's damage - especially since their lower range also limits their position relative to the target they want to attack. As such, melee champions demand a lot more stat-based durability, which is expressed in higher base stats. And since base stats scale with level, melees are worse off when sharing XP than their ranged brethren. Especially given how base stat tankiness tends to at least triple between levels 1 and 18.
The other part lies in the auto-attack focus of these ADCs. In contrast to ability-focused champions, the increases in auto-attack efficiency are far lower with additional levels than the increases in ability efficiency. In addition to increased base damages, abilities can also gain increased utility, decreased cooldowns or even increased scalings. For the same effectiveness increases in gold, a champion would need to pay quite a price (for instance, an increase in 40 damage in an ability that scales 0.7 with AP would require 57 AP, which is almost a NLR at 1250 gold - for a single level-up). Contrast the effect of level increases on auto-attack damage: The most damage a champion gets for a single level is about 4.5 AD, or 162 gold. This effect is even more pronounced for ADCs, whose base AD increase between levels 1 and 18 is seldomly even enough to double the initial value - and, for most of the class, is not significantly over 40 AD. 40 AD, which is the equivalent to a single BF sword at 1300 gold, for 17 levels of base stats. Compare the above value for a single additional rank in an ability.
So, auto-attack focused, ranged champions scale well with gold, but proportionally less affected by getting less XP than other classes. Of course, additional level never hurt, but if one were to decide between getting an additional level on an ADC and on a tank/mage/assassin/Juggernaut...
With that established, how does it affect the rest of the map and general champion variety on a coordinated level?
By always having a ranged, auto-attack based champion on the bot lane, teams will have a solid foundation in the sustained damage and (usually) physical damage departments. As this is the case, priority on these two departments decreases for the other roles.
This negatively affects the viability of sustained damage focused champions in other roles on the coordinated level, as bringing something like a Skirmisher or Juggernaut will mean that your composition has a lot of physical damage, but will be lacking in other important team-building departments. For the two classes mentioned above, bringing a Skirmisher would usually result in a hit to fight control, tankiness and/or engage ability, whereas bringing a Juggernaut results in a loss of engage ability and an increase in kiteability. As such, these kinds of champions are mainly restricted to niche roles, when their specific strengths are suitable against the enemy/with your own team or their weaknesses do not lead to an exploitable team comp. Or they are numerically so overtuned that the weaknesses don't really matter.
How could that be changed?
A good solution to this particular problem will be difficult to find without changing the game utterly. An approach (that would render the game less fun, imo) would be to make XP sharing grant both partners the full value (or even distribute earned XP among the whole team). That's what Heroes of the Storm does - and it is a vastly different game for it. Another approach that might be more feasible would be to design champions (not items, but specific champions) that are of a non-ADC class, but that function equally well when sharing XP. And, and this is the crux, that are not too powerful if they get actual solo XP. A good example for a potential mechanic for such a champion could be found in the first reworked Cass passive: This passive would give her boni (at break points, which is less optimal) if she had accumulated enough stacks. And she would gain stacks by applying poison to enemy champions, making her even better in duo lane: More targets to poison = more stacks. Now, in Cass' case, this would result in a magic sustained damage champion (still filling the need for sustained damage), but I can't see a reason why a similar system should not work for a burst mage.
In any case, League would need a few of these non-ADC XP sharers, or teams would be forced to early-pick the existing one and thus lose a lot of flexibility on the draft.
TL;DR: Resource allocation makes ADCs the optimal farming botlane choice. Having a source of sustained physical damage makes additional sustained damage dealers less attractive, lowering their viability. To overcome this, League needs working non-sustained damage bot laners, which are difficult to properly design.
