Adding the option to purchase/unlock costumes for heroes you already own.
Let’s face it most past heroes with a costume are no longer in the top meta (especially the past HOTM/event heroes with a single costume) such that the big spenders/P2W would rather pull for Owl/Goblin/Star Elf/Construct heroes than chase costumes for past heroes. And for C2P players if they have already pulled event/past HOTM heroes before it got a costume they are unlikely to have the good fortune to have lightning strike twice to pull it again for the costume.
However it might be a good investment for C2P players to be able to pay a small fee - say $10 for a 3* costume, $20 for a 4* costume and $30 for a 5* costume - to unlock the costume for a hero they already have. A guaranteed costume buff to a hero they already have (and maybe levelled) would for many represent a worthwhile investment over spending for a small number of pulls at low odds gates.
And for heroes with multiple costumes SG/Zynga could sell them separately to be able to make $90 for a hero with 3 costumes to unlock.
This wouldn’t completely invalidate having costumes in summons gates for this only applies to heroes you already have so if you don’t have it you can still summon for it and get the costume free with the hero, or for the costume summon some people may want to see if they can get the costume for free with their costume keys.
I think this presents a fair balance for the developers to still make some money from costumes but also make the costumes more accessible to the C2P players that may still be relying on those heroes (that could use the buffs) as the whales are chasing newer heroes at different summons gates.