Some of them have already started accepting bitcoin, in future other established altcoins will also be accepted such as Ethereum, Litecoin, and Neo.
Out of the few stores i have seen that accept bitcoin they seem to accept litecoin or ethereum too. normally they accept 2 currencies bitcoin and then 1 other.
This is because most of the people that are accepting cryptocurrencies are enthusiast and decided to support this field by accepting cryptocurrencies in their business, so they will accept bitcoin which is the most widely circulated coin and some other coins, most likely the ones they have some preference, after all even if I believe bitcoin is the best coin around that does not mean I do not support other coins.