This would be super useful.
As an input, this kind of thing would be helpful:
20USD in yen
£5 in yen
CAD199.99 in yen
EUR50 in yen
15,000円 in USD
Here in Japan we actually don’t have a decimal point in the currency (except in currency trading and the stock market), so some rounding might be handy. Although I’m used to doing it myself. 
How about a user specification as a fallback if there’s no ‘to’ currency specified?
Like, if mine is set to yen, this:
USD20
…would always result in a conversion to yen (hopefully with rounding), whereas this:
¥2000 to USD
…would convert to the specified ‘to’ currency.
Hmm, it would also need to know that when I say ¥ I mean yen and not yuan (RMB)…
Or even sweeter:
2000 to USD
…understanding that I want to convert from yen, because that’s my specified default.