I don't think massive systemic changes are actually needed all that much to make ADC/Support bot lanes not mandatory. Sure, there are some systems changes that could help, like working on creep block and stuff. But I think that the real way that ADCs stop being mandatory, or at least mandatory in the bot lane, is reworks to both ADCs and Supports.
Lets look at something interesting:
Four of the six champions that were reworked as part of the Marksman update are now four ADCs who are pickable outside of the bot lane ADC role:
Graves - Jungle
Quinn - Top
Corki - Mid
MF - Support (I know this is a niche pick but stick with me here)
What's evident here is that when ADCs are changed to have unique reasons to pick them outside of just sustained physical damage, they become viable in other roles, because there are now reasons to pick them there. What other ADCs get picked outside of bot, and for what reasons? (not counting AP builds here)
Vayne - top - tank killing
Varus - mid - supreme physical damage poke, counter-engage
Twitch - jungle - stealth cheese ganks, later teamfight AoE
In addition to these, you have one champion, Kennen, who isn't an ADC but is occasionally picked as one because he brings something unique for the role - consistent, often AoE hard CC.
So if its possible to have your team's source of sustained ranged physical damage come from outside the bot lane, why is the bot lane still always an ADC?
I think the real problem is the supports.
Think about it for a second - almost every dedicated support is built around either enhancing specifically the patterns of a ranged ADC (Janna's shield gives AD, Sona Q gives an on-hit, Braum passive procs on autoattacks) or stopping the patterns of a diver or assassin (Lulu W/R, Janna Q/R, Soraka W). I think the problem with botlane might not be that theres no one who can replace an ADC, or that the mandatory ADC can't go anywhere else, but that there are no good lane partners for a non-ADC bot laner, and that any support you'll play against is specifically designed to fuck you over.
This might be entirely wrong but think about it a bit.