Buff Bloodhound, Mirage, Octane and Crypto

I agree with the OP. Although I think out of the four legends he has mentioned, Octane is probably in the best spot.

Here are some basic ideas on how to buff them:

Octane: Either do something with the jump pad (like give speed buff to the teammates when they use it), or make it so he doesn't loose health when boosting. That's about it.

Crypto: Greatly reduced drone sound, and add some sort of follow mode, so you are getting benefits of extra info about enemies while still been combat capable.

Mirage: Decrease cool down time on his abilities greatly. Make the clones look and act like the player, as in hold the gun like a player, and actually give them basic AI so they can jump, slide and climb over a wall. Maybe add another passive perk that creates a clone of mirage automatically whenever he takes damage with cool down of like 3 minutes or so. Mirages everywhere!

Bloodhound: Reduce or completely take away the sound when he activates his ultimate, and also while he is using ultimate allow him to climb walls just like Revenant.

These are pretty generic and can be refined further, but at least they are a good start to get those legends to somewhat competitive level with the rest of the beasts we got atm.