I know that 6* characters have either a leader or specialist skills, and I have thought about it, and it seems pretty logical to me that all 6*s should deserve both a leader skill and a specialist skill, be it good or bad.
Firstly, we all get to see new teams, new faces if all legendaries have leader skills. Not just mirabelle and carl teams, but siddiq, wyatt, yumiko and so much more. I get that some characters are already strong, but certain amounts of balancing are still needed.
We all understand that 6*s are new and a test to this game, so a few tweaks should be a must to allow more fairness in the game.
Personally, I especially feel that Carl and Erika is too strong for their skills coupled with their leader skills. They outshine all other legendaries, like jeremiah, hershel and others.
Well, because of this, less people is going to get these characters and the character pool ends up being saturated with Carl and Mirabelles. I bet you all have went through many rounds of testing for each character, and we are all excited to see a new face every time. Kudos to your team. But, do try to make it such that users would want to get it, for example how Erika was a thing in my region and faction.
Saw a photo of Rick, love its specialist skill, would be better with a leader skill too, honestly. I want to see other players using him as well, would be a cool thing.
Off topic: I suggest having a 6* draw fest to draw the old characters again if you are planning to change them. Gives chance to new players too. Maybe you can allow 10 5s for an ascendable 5, as that will be awesome. Once per player.
TL;DR :: 6*s should all have leader skills.Since they are the best, you know.