Instead of the search bar to search for a champ you organize the champs by role, for example when someone selects marksmen, it'll only show champs that are marked as such. It sounds lazy, but I would think it's more of those time when you want to play a role, but you don't know who and you just want to look at all the champs you own in that role to decide, at least that's how I feel sometimes.
What about Support? Because Leona and Blitzcrank and a bunch of mages won't show up like that because of their listed Primary role. Or if you're going to play something off-meta?
There are great applications for your proposal (in all modes), but needs to be tweaked properly to accommodate these things.
My suggestion is to reorganize the Roles so that people like Blitzcrank (a Tank/Fighter) and Leona (Tank/Support) have their roles listed as what they're actually being used for, and an add-on to what you want could be that filtering a role lists all the champions that have that as their Primary followed by all the champs that have it as their secondary [example below];
"Searching for Tank"
Primary Tanks
Secondary Tanks
I'm tired and just rambling at this point lol. But the point is that listed roles don't always fit the meta, and non-meta picks can work too, plus the filter needs to accommodate searching for secondary roles.
The idea of having an extra tab between the skins and the champions tab while in champion select has been brought up numerous times. Although Riot has never commented on a favorites tab yet :(
Basically you could put whatever champion you want in that tab and you could also label for subcategories. So you could have a list of your "TOP" champs or a list of your "BANS".
