Oh, I've seen Darcy/Tony, but it's more usual that she's his kid. Either that or she's Coulson's kid.

Oh god, the one where she's Tony's child *and* a genius who built her own AI... never, ever click on that one.

There's a couple where she appears because she's been shuffled into the back corners of SHIELD for exactly that reason - also, she's a political science major, so SHIELD might well have an interest in her anyway.