If you really want to fill it up then...

Waves for W: https://coinmarketcap.com/currencies/waves/
Qtum for Q: https://coinmarketcap.com/currencies/qtum/

I actually think that Powerledger is a stronger bet than the existing P crypto because it is one that will allow people to trade excessive energy that are captured by renewable installations and has a pretty strong future, but of course you have to make the decisions if there are more than one with the same letter :)