With mentioned earlier you can change only other market pairs. "id" is required but whatever i put there it works. For different data you'll need other method:
curl --data '{"jsonrpc": "2.0", "method": "get_order_book", "params": ["OPEN.STEEM","BTS", 10], "id": 3}' https://bitshares.openledger.info/ws/
But this is far from optimal, when comes to real time ticker.
I was thinking about something like this:

1. take markets data from Poloniex
2. Calculate BTC prices to BTS using highest Bid or last price
3. Take markets data from DEX and compare with above - indicate arbitrage opportunity.

Why? Because those BTS prices talks nothing to newbies and they are maybe to lazy to calculate.

But Cryptofresh is updated hourly so not good for "real time" data.
It seems RPC calls is not best choice and also very limited.
With full node you can obtain any data you want, but this requires... full node.
We need server with better public API (just pure data), so people could build own stuff with minimum effort and skills.
*note: I'm noob