What Riot is suddenly doing right with champion designs
I'm going to illustrate my point with supports, because nobody listens to support mains anyway because they demonstrate my point well. If my point is wrong, they'll demonstrate how I'm wrong well.
Sona and Nami are supports. Nami has better CC, Sona has better poke. Both have unique kits, both scale differently as the game goes on. They're not identical.
But if Nami is weak- and I would argue she is right now- people will pick Sona instead. If Sona is weak, people will probably pick Nami instead. There is usually room in the meta for that type of support, but the actual support people pick will change depending on which one is statistically better.
Now take Leona. The meta will want different things from a support, depending on what team comps are strong right now. Some seasons, people will pick Leona. Some seasons, people will pick Sona and Nami. These two sets of champions are competing.
Now- plot twist- take Bard.
Bard is different. Why is Bard different? Why is he less replaceable? It's not because he's a support with a unique kit, although he is. It's because he's a roaming support. Notice that map terminology. Bard is unique not because his kit does different things from everyone else in his role, he is different because of his position on the map.
This is a common trend all of the sudden, and it's really exciting.
For example:
is a jungle champion. She is strongest in tight areas with lots of breaks in vision and many short walls. In all other regards, Rek'Sai is basically just another melee carry, or another bruiser, depending on how AD itemization works at the time. But because she has a role on the map, she's unique.
is a siege champion. He is strongest in the spaces between turrets, because he can control every single exit as long as he can put up a turret. He's not just another mage, because he takes control of a specific spot on the map. You can't replace Azir, because as many mages as there are, there aren't any that are specifically designed for the same spaces.
is the same way. Kalista owns the bottom lane. Not the laning phase, or the meta in the lane- I mean the lane, the position on the map. We're very used to thinking of "lanes" as being their own metas, rather than actual spots, aren't we? Sentinels scout out all of her blind spots, she has significantly more mobility options around creep waves, and she's designed to stick to her support.
If every champion is like this, we can get an entirely different game. Every champ is less replaceable, because they have so much more room to specialize. People are free to make their own meta. The almost chess-like feel of LCS matches is amplified to a ridiculous degree as players maximize the positional advantages of their teams.
Please, please don't make this a temporary trend. We need more Bards.

Is a fairly new rework with this same strength. Azir is more mobile, while Heimer is better at prolonged Seiges of a single tower/objective. Generally, whatever the donger sits in front of, he's going to get unless you take steps to remove him. and that becomes... interesting.