<Snipped quote by Lord Wraith>
Maybe. Though of course, we want people to feel free to take up any characters they want as long as they're no other players taking those roles.
Maybe it would be better to have a list of characters that are already taken and then one of some suggested characters?
And maybe figure out things regarding characters we love to have around and want to keep the in-universe lore of, but don't seem like they're going to anywhere. For instance, if Supermaxx doesn't come back for him, I would still want his take on Wally West to be part of the game, mostly due to how much care was put into his background and how it connects with Sep's previous Flash!Iris. Then also Bounce's Aquaman just in case she's able to come back.
You get into an awkward position with all the characters already taken, then you have all the IC characters introduced via player supporting casts. And then to cherry pick what we want to remain canon from being dropped it gets really overwhelming for a new player.
And I'm speaking as someone who has been trying to get a new player in the door.
Long story short, to do a recruiting drive properly, you're either going to need to give some guidance, whether by documenting all characters currently in use, or give concession in that new players are going to need freedom to break from previous posts by dropped players.
If that last idea rubs you the wrong way, then you need to seriously consider if the game needs a recruiting drive.