I'm sure they will add more trading pairs in the future ALTCOIN/USD.
But Bitcoin and Ether are now the most interested onces to start, look at etoro.
I understand why they will stay away from other smaller currencies for the moment, the risks are so huge. Invest small and the returns will be small, invest big and your returns are big! Thats how the normal stock market works , this is just not possible in the cryptoworld (NOT YET).