alt Hacker NewsFree parking, turn the centre into a relaxing space/park, with kids' playground.
Add coffee shops and restaurants nearby, help promote casual entertainment venues like mini golf, community theatre, and weekend markets.
Grow from there based on the type of demand that appears, keep business rates low for the area.
They say it can't be done, but when that last shop closes right now there will be no choice.
To combat the dereliction and anti social behaviour they would have to rebuild anyway, and it will cost them more later than gradual repurpose/ investment.
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I'd replace the free parking with great public transport and cycling infrastructure. The parking spaces can be in the surrounding area, with frequent public transport between them and the centre.
Look at European cities that have implemented this even in part, and tell me it isn't much nicer with fewer cars. Shopkeepers are usually opposed to it before, but see that the improvement in atmosphere attracts more customers than are lost through the lack of parking spaces.
This is it, and I have heard politicians talk about these same ideas for years in my area. Alas, it looks like it's easier said and done, because the downward spiral is still ongoing.
And one of the reasons is definitely that out of your list, city planners have very different ideas about the first point, free parking. They want the opposite: ideally, no cars in the city at all. So, parking fees have been raising to exorbitant ranges as they want to incentivize people to take public transport instead. Except that people instead turn to internet shopping.