" I doubt Riot will actually do this, but what do you guys think? Any interest?"
In regards to the spawn ceremony--
Remember that emotes used to be temporary flairs during events too. I distinctly remember the original VS. event having emotes that could temporarily replace your mastery emote.
Now look. We have a ton.
I imagine if they gain enough popularity there's a good chance Riot inevitably monetizes them and makes them permanent.
Finishers are a bit more iffy imo, we had the finishers for events like URF and nexus blitz, but they, at least in my opinion, tend to be sort of flashy and distracting which has the potential to disrupt gameplay.