Idea 1: Cas' true form gets inadvertently turned corporeal so Dean can see it without his eyes burning out, and he really is the size of the Chrysler building.
Idea 2: Canon S14 Cas and Dean get arrested after defeating Michael and everybody, and after everything that's gone down, it just seems like not a big deal at all, so they chill out at the station while the cops around them are all freaking out about their rap sheet.
Dean goes to a flower shop because he wants to bring his sister in law something special for her birthday (cause she is awesome) and literally runs into Cas as Cas is browsing a selection of flowers appropriate to send to his boss' wife at the hospital after having a baby. They can either exchange numbers, or plan a date right there (and exchange numbers, of course)
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Idea 2: Canon S14 Cas and Dean get arrested after defeating Michael and everybody, and after everything that's gone down, it just seems like not a big deal at all, so they chill out at the station while the cops around them are all freaking out about their rap sheet.
Idea 3: time travel (any kind, any direction)
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They can either exchange numbers, or plan a date right there (and exchange numbers, of course)