I've noticed that the number of Symphogear crosses seems surprisingly sparse, so here's some thoughts for possible crosses. WARNING: Symphogear spoilers may appear
First, Ranma:
Ranma was raised on the road visiting many exotic places, so there's plenty of opportunity, especially given Genma's larcenous habits, for him to come in contact with a Relic, possibly an incomplete one like Tachibana's, and from there it's just a dunk in Jusenkyo and an encounter with Noise for Ranma to potentially have an active Symphogear. On the plus, Symphogears seem to be very physical compared to stuff like Sailor Moon, on the minus, she has to sing to power it. Also, the Commander is basically a superpowered martial artist in his own right, so that could give Ranma further incentive to stay around.
Second: Nanoha
Simply making the city Symphogear takes place in(which... I don't think is ever actually named) be Uminari city, and from there you can have the gambits of Precia and Fine overlap, causing many complications and interactions that could be quite interesting.
Third: Fate/Stay Night
Hibiki is, arguably, almost as messed up in the head as Shirou is(admittedly depending on viewer interpretation), and Shirou would be compelled to help, or attempt to, with the Noise problem. Bring various Magus complications into the mix, and depending on the chosen point of view for presentation, it could also be quite interesting.