I'd probably say "in future, could you please refrain from eating loudly" but "in the future, we will have flying cars". "The future" would refer to the magical world full of fancy new technology, while "in future" is more along the lines of "from now on".
"I'm in hospital" makes perfect sense but "I'm in the hospital" works perfectly fine as well. I don't think either is incorrect in English.
Regarding the sentence construction, I wouldn't understand "a great car, this one" as a standalone sentence. Written, it'd have to be "it's a great car, this one". Spoken, I'd expect there to be either something spoken prior (e.g. "Isn't she lovely. A great car, this one.") or some kind of non-verbal cue (like a look of pride or fondness that essentially expresses the same thing).
But again, Australian, not English.