Montana James has the ability "Damsel in Distress", which boosts the DPS of all crusaders if the Princess is in the formation. The latest Trek Through the Hidden Temple Event added Danni, the Daring Damsel, and it seems like she'd also be a good candidate to activate this ability.
It would be nice if having either the Princess or Danni provided the DPS boost, although of course you should not be able to get double the boost by having both of them.
I think this small ability buff would be worthwhile for the additional formation options, and seems thematically appropriate.

This is not going to happen. Have you not noticed Danni's "Daring Savior" ability?

Hmmm, yeah, what Andreas pointed out. Danni's theme is stated quite plainly in her "Self Sufficient" ability. Quote: "I can take care of myself out here."

Plus, as MrWizard mentions, Danni has a synergy effect based on Montana, with a quote indicating that she has a crush on him. (Well there's a love triangle, if Montana has his eyes on The Princess.)

I do sometimes think though that it would be nice if some crusaders could be slightly more open-ended with focused synergy effects in their abilities or legendaries. Meaning that maybe instead of referencing one specific crusader whose presence allows them to activate a bonus effect (which makes sense for specific pairings but it's really restrictive)... Perhaps for some crusaders the reference could be for a specific combination of multiple tags?

Like for example, Leerion's legendary coin currently references Sasha (love triangle? because Jim and Sasha are a couple according to the Bush Whacker games and Jim has that effect on a legendary as well) but suppose that Leerion's item instead referenced female+human+support tags, which provides a pool of 10 options which can activate that buff (Pam, Sasha, Mindy, Danni, Princess, Momma, Red, Siri, Shadow Queen, and Agent 79). (Buff is on/off, of course. Would only need any one of those characters present to activate it. Multiple would not give extra bonuses.)

Or there's Jason's legendary goggles which currently reference Emo. What if that instead referenced male+animal+DPS+supernatural, which would open it up to either Emo or Frosty being allowed? Or better yet, maybe just male+animal+DPS so that it's Emo, Frosty, Draco, Sal, or Robbie for that buff?

I suppose something that's comparable that is already on a few crusaders is that there are some with conditions that an ability only activates when there are more of X tag than Y tag (more female than male, or vice versa). So it's not tied down to just one specific crusader's presence and there are instead many options to satisfy the conditional check (and the pool of options can expand with the release of new crusaders without needing to update the conditions).

Yeah, I was thinking of RoboQueen a bit there too.

And then later on after posting that, a recipe mission allowed me to get Red's epic plane and then I see that the legendary effect for that would be "increase GDPS when there are 3 or more female humans". Double tag check -> Currently 15-16 crusaders to select from there, depending on whether she counts herself. So I guess there are already at least a couple examples of them trying out some slightly more complex rules.