I realize that realism isn't always this show's strong suit, but I'm not excited about any surprises about either Charles or Grace's paternity. Given the way family law works in the US, both Charles and Grace are old enough that established relationship trump biology. If the idea is to explore the complications of Danny being a single dad to two children (one fairly young), the wouldn't need paternity drama. If something happened to Rachel and Stan, Danny would raise Charles because he's Grace's brother. I'm not even the biggest Danny fan, but that would extremely consistent with his character.
I'm a McRollins fan and I want some resolution. Either end things or figure something out. They could have Catherine go back to the Navy and work in the occasional reference to them talking/emailing and taking time to spend leave together. We wouldn't even have to see it. I just don't like the limbo.