4. The Adventures Of Captain Jack Harkness (Torchwood/Doctor Who)And River Song (Doctor Who)
I'll admit my next choice is debatable as to whether it really is a TV cross over as these two characters have been in the same show. But they have never been in the same scenes, despite their appearances on superhero TV series
Arrow and their shared vocal agreement that their respective
Doctor Who characters need to work together. I've been waiting for a meeting between these two for some time. They best place would back in the days where Amy and Rory were still on board the Tardis. The flirtation and innuendo would ave been hilarious. Jack and River flirting. Jack liking the new Doctor (He hasn't met Eleven). River announcing that he is her husband. Amy liking Jack. Rory objecting. Jack liking Rory. The Doctor announcing that they are River's parents. I can see it now. But even though that ship may have sailed, we still need these two characters to meet up and go on some exciting adventures together. Their connection to the Doctor(s) would make for a interesting dynamic and these two aren't characters who will play by the conventional rules. Perhaps this could be the natural successor to 'grown up'
Torchwood if there ever was a potetial for another
Doctor Who spin off. Their darker personalities and sense of fun would make for some iteresting adventures. But frankly I'd be happy for some shared scree time on the show they first came from. Because we all need some more Jack and River in our lives.