Yes that would be nice not too sure of any dates or anything yet but they will have one for bonfire night. It's normally on a friday night I think so could meet up beforehand to have a quick chat, and then go watch the fireworks which are normally on about 8 and go for a drink after.
One thing I will say though is we'd have to meet up about 6ish to get any sort of parking within a 5min walk of the Castle. Either that or (this could be controvertial) we could watch the fireworks from up on Caerphilly mountain, though I think the atmosphere is better within the Castle