Here's a suggestion for ya.
How about implementing secret unlockable crusaders in which you have to do something specific. For exapmle sending Sisaron the Dragon and Brogon, Prince of Dragons on a mission and you get a baby dragon crusader or how about playing through a certin existing objective with just robot crusaders and you get a crusader named The Maker.
Possibilities are limitless.
Or instead of unlocking crusaders you can unlock permanent Missions that are better then your standard everyday missions.
Of course unlocking conditions would have to be equivalent to the reward and vice versa.
So what do you think DEVELOPERS?
I would rly like to see something like this in the future. Would also be better if unlocking conditions were different for everybody that way no guides can be made and it would make it a whole lot interessting. For me atleast.

It wouldn't even be people discovering it in regular play and spreading the word. The dataminers would uncover it in the data added in the background of a patch (when they aren't even going to be accessible until a later patch) and they'd start spreading the word before the crusaders are accessible (granted, it would stick to being a rumor passed around in chat before release and it'd only go up on the wiki / fansite / etc. after it's released).

Anyway, if the game had never gone down the path of tying the vast majority of new crusaders into time-limited events (and later the RNG alternative of recruitment missions) though, I could definitely imagine a version of the game packed full of obvious and not-so-obvious recruitment objectives and "secret" conditions. Like each campaign would have at least one tier 1 recruitment objective (maybe even two or three of them) that's scaled a lot easier than the ones we know, and then there would be additional recruitments in each campaign for the upper tiers as well. Plus this topic's secret conditions idea on top of that.

But what we're actually getting for non-event unlockables is a lot more limited than that. If they were to consider taking something from your idea, it probably wouldn't just be "send a certain pair on a common 2-person mission to unlock someone". They'd probably do something more like "send a certain group on a rare or epic mission (required groups for each unlock are specifically chosen to have at least some mismatched tags for the mission which guarantee that 100% chance cannot be achieved, and the mission must succeed for the recruitment to work, although at least the group would be chosen within some logical theme hinted at in the mission description)".