NXT already reached a $100+ million market cap twice before and both peaks occurred 6 months apart and during a long term bear market for cryptos. Whether or not NXT reaches $1 per coin would depend a lot on the BTC price since the prices of altcoins usually follow the BTC price as it rises and falls. A $1 billion market cap for NXT sounds like a very optimistic prediction, but if Bitcoin's market cap is $50+ billion then it's not unrealistic for the second, third, fourth, and fifth largest cryptos to have market caps over $1 billion. Whether or not NXT will be one of these coins is unknown, but out of all the altcoins out there, NXT is probably one of the more promising ones. It's technology and features are innovative and it has one of the most developed ecosystems of any 2.0 coin due to its asset exchange which has more users than any other competing platform.
So to answer your question, I think Bitcoin's price would need to rise again before we see any altcoin with a $1+ billion market cap. So far, Litecoin was the only altcoin that was able to reach a $1+ billion market cap (Auroracoin doesn't count) and it took a massive spike in the BTC price for this to happen.
How much is it today?
It's been hovering around 1 cent for a while now. According to CMC, the latest price is $0.009341 so it's slightly under that now:
http://coinmarketcap.com/currencies/nxt/Nxt price is still much coupled to Btc's price, like most other alts, when Btc goes down or up Nxt follows too. On the other hand it has been steady among first 10 crypto and has an incredible pace of development. The asset exchange is one of the best features and there are a lot of assets but this seems to have had little effect on price. I believe things could change when the SuperNET goes live, Nxt is one of the core coins and I expect that its price will begin to decouple from Btc.
NXT is currently falling a little bit faster than Bitcoin. Just recently the price slipped under 4,000 satoshis for the first time. However, most altcoins have been losing value even faster than NXT so it actually managed to climb up the rankings despite this.
whats a scenario if a coin were to decouple from btc?
Decoupling means that the price would no longer be determined by Bitcoin's price. Litcoin's value decoupled with Bitcoin when it began to lose value much faster than Bitcoin did. A coin can also decouple if it becomes big enough that it becomes independent of BTC. If NXT were decoupled from BTC, it would mean that NXT's price fluctuates independently from that of BTC's.