There are rumors that Polo plans to add a few new coins once they delist the 17 coins they plan to remove. The delisting is June 1 I believe... so watch very closely what happens the next few days!!
Actually, the delisting will happen on May 2:
https://cointelegraph.com/news/poloniex-to-delist-17-altcoins-in-may-without-explanationTherefore I'm stocking up on PIVX at the moment as much as I can.
PIVX is the only coin among the first 20 by either market cap or volume that isn't on Poloniex, and there will be room for 17 new ones! Draw your own conclusions.