[reason 1],再说 + [reason 2]
[time / condition] + 再说 (吧)
再说 at the head of a clause adds another reason for the same conclusion: "besides", "and anyway". The first reason comes first; 再说 brings in the second. It is spoken.
The clause after 再说 often has 也 or 还, since it piles on top of what came before.
At the end of a sentence, after a time or a condition, 再说 means "we'll deal with it then". 以后再说 is "let's talk about it later", often a polite way of not deciding now.
再说 adds a reason in the same direction. It doesn't introduce a contrast, and it doesn't mean "then, next".
请再说一遍, "please say it again", is plain 再 + 说, a different structure.