I really like the thought of Castiel showing off his angelic strength and manhandling Sam around, maybe something with that in mind? Maybe a plot where some witch or something curses Sam to be mated to Castiel, or tethered to him in some way. And the two of them are forced to find some sort of solution to undo the tether, but the effects of the curse/tether/mating/etc are too strong for either of them to resist for long and they realize how perfect they are for each other?

Alternatively, a plot where Sam has been pining for Cas since he first showed up but because of Cas' original tie to Dean, Sam feels neglected and forgotten about. Cas will come when Dean calls, but never come when Sam calls, and most of the time it seems like Cas sees right through him. Maybe we can do something where another hunt goes wrong and they end up getting cursed in a way that makes it so Cas can read Sam's thoughts, not just his prayers but all the sad, dejected thoughts he has when Cas doesn't look his way for long or his pining. So, when he, Sam, and Dean are stuck on another hunt together, Cas tries desperately to ignore the way Sam's pining thoughts are screaming out to him, but it becomes increasingly harder to do so until he lets it slip that he's been able to hear what Sam is thinking for awhile now. And then poor Sam is so embarrassed from it but they work it out!

If either of these sounds interesting, dm me on discord! my username is kazikuns

I'm also open to a lot of other supernatural ships so don't be afraid to ask if I like the same rarepairs as you!