Thanks JD88, great thread. As more people start to live in city centre apartments, it will be increasingly more important to have green spaces in the centre. The irony is that as long-standing undeveloped sites are become more attractive to developers (a good thing), the potential space to develop genuine public green spaces starts to diminish (er, not so good).
There is currently a campaign on change.org led by citypark4brum to ensure that a park is integrated into the Smithfield development. They are trying to attract 5,000 signatures, and have around 4,800 atm, so worth signing up if it's something you're interested in.
There is also the Skypark proposal to create a linear park on the disused Duddeston viaduct in Digbeth, that should hopefully come about on the back of HS2-led development in Digbeth. This is in the vein of New York's Highline, and (more appropriately imho) Paris's Viaduct Des Arts/Promenade Plantée/Coulée Verte.
Personally, I would like to see Highgate Park extended down to the River Rea whilst there is derelict land there. We've seen the housing application for Moseley Street this week, and with other housing developments in the area the population is set to rise around there. I love the idea of a canal-side park in central Brum too, must be loads of scope for that in the central area.
I agree with others that tree planting in the centre makes for a more civilised and healthy environment so planting more trees where appropriate always a good idea.