Sometimes they will temporarily bring back removed stuff, like they might bring back some old Christmas stuff during that season. But you can't tell what will and will not come back.
Seasonal crops are removed whenever teamlava decides to remove them. 2012 seasonal stuff was removed in jan 2013. For other games Halloween 2013 stuff was removed last Friday. You don't know when they will be removed, but they will most likely be removed eventually, so when seasonal stuff is released many focus on mastering them before the other crops.