I think a major problem with Riot's balancing over the years is that it's always been fixed at some arbitrary cutoff point. We've frequently been told that Riot only balances for Plat and above, and that below that it's either too difficult to balance the game to be worth considering, or simply that the best balancing at that point is simply for players to play better (this is literally what at least one Rioter has said). As such, very little effort has been made to balance the game at lower divisions, even though a ton of work has been done to balance the game around the incredibly tiny LCS population, at the cost of everything else. This kind of elitist and narrow balancing philosophy has caused the game to remain in a very unbalanced state for the vast majority of the game's players, as Plat+ represents only 10% of the playerbase, and the total pro population less than .0001% of all active players. This is one reason why the game's balance attracts so much criticism, because most players are simply not served properly under this methodology. We all joke about how Gold and below players need to "get gud", and some people on here even consider that a valid argument, but the reality is that if a game is unbalanced for the near-totality of its playerbase, it is simply not a balanced game, period. As such, we need a balancing philosophy that caters to all divisions, instead of stopping at a very small sample size.
The thing is, at this point it's become increasingly clear that there are some mechanics that are reliably stronger at certain elos than others. Any mechanic that scales with communication, for example, has a disproportionately high impact in pro play compared to everywhere else, which is why champions like Shen or Ryze are competitive nightmares, even if they're often shit at any other point in standard play. Comboes with difficult mechanical executions also tend to scale better with higher elos, as well as ping. By contrast, features such as proximity checks on abilities, or low mechanical complexity, tend to do really well in lower elos above all others. Even Riot has acknowledged this, and from time to time we've seen them use these lines of reasoning to equalize a champion's balance across a wider spread of mastery levels, as they did with Lee Sin and his energy returns a while back. Effectively, it is possible to balance champions across more elos than just Plat+, if not across all elos, so the fact that this isn't more widespread simply comes down to Riot choosing to prioritize other things instead. At this point, it is not so much a matter of figuring out how to design and balance for lower elos (though there's certainly a lot to explore there), as it is to actually motivate Riot to do so in the first place.
Part of the problem here is that the very large majority of Riot's recent releases have been enamored with high mechanical difficulty, and just high skill floors in general, despite the fact that a champion's learning curve should ideally have a low skill floor, i.e. good approachability, and a high skill ceiling, i.e. plenty of room for mastery. When Riot tries to make a champion easier to learn, the designer frequently forgets to do both, so we're often stuck with champions that either have far too low a skill ceiling (e.g. Garen, Shyvana, Malzahar...), or far too high a skill floor (e.g. Shen, Ryze, Kalista, Azir, etc.). Only rarely now do we have champions that can do both, with the new Warwick being the only real recent example of a champion that is truly easy to pick up, but also fulfilling to master. It might be that the issue here might be that the elitism driving some balance decisions also has an impact on champion design, so that certain designers genuinely believe that making their champion useful only at certain elos is a good thing. At this point, we have far too many designs that try too hard to show how mechanically skilled one needs to be to do anything useful with them, and far too few designs that make any real effort to approach newer players. Starting with that, and baking in a mix of different mechanics that allow a champion to be tuned around more elos at once, could be a good starting point for Riot to broaden their balance to the rest of the game's players.
