With this in mind, selling something that is of little use to get something really useful would be a step forward.
97,780 passengers, and counting, would object to it being of "little use". And the "really useful" in this case would be trading it for a decked car park across platforms at Marple? Thank god that kind of 40-year old thinking is no longer shared by transport planners!
The fact that Rose Hill is a relatively quiet terminus isn't an issue, as long as the trains fill up by the time they get to Piccadilly, which they do. With the potential for tram-train to Manchester and a link into Stockport town centre, Rose Hill provides the opportunity to convert so many more car journeys into public transport.
I like the idea of a simple shuttle bus going Rose Hill - Marple centre - Marple station, like our own little Metroshuttle linking both stations into town (although I suppose the 383/384 does this already), but I think you'd find that closing Rose Hill and providing a shuttle bus to Marple wouldn't work. No-one would make that journey from Rose Hill, they'd just get in their car and drive. Welcome back to the 1970s.
Not a cat in hells chance . Of a deck on top .
I'm not sure how else the parking could be expanded. As Barbara says above, it's effectively a park and ride for people from New Mills and all around, and those park and ride projects seem to get a good amount of funding, if that's the issue.
Although I wouldn't like to see it as a priority myself... Given Rose Hill often has spare spaces to Marple's overflowing overflow, a far simpler solution would be to advertise the improved Rose Hill service at Marple (and to continue improving the Rose Hill service), encourage a few people to switch across first.