@SmokedDuck
I'm not convinced that an online retail sector is a great thing for cities or the people in them. If there is a plan to be made I think it should include reversing this trend as much as possible.
But it's only a recent thing that city centres were dominated by retail. You only have to go back 20/30 years and there was a wide diversity of businesses, including GP surgeries, vets, warehouses, factories, print works, breweries, with lots of homes and lots of small shops dispersed around, as well as the department stores, chain stores, etc.
Retail pushed all that out and now bricks and mortar retail is is massive decline, it's leaving a vacuum. The OP is correct, we need to re-purpose large areas of town/city centres as High Street retail won't be coming back. For non-touristy towns, common sense would suggest homes to reduce commuting etc., but that also needs local employment, leisure, etc. For touristy towns, then they'll already have hotels, leisure, restaurants, theatres, etc.
What would be good would be for the big national "office based" firms, like banks, insurance companies, solicitors, accountants, etc to open regional offices in smaller cities and towns, i.e. reverse their centralisation into London, as they used to have smaller offices. Then their staff can work closer to home, and regional people would have a chance of getting decent jobs in national firms without relocating - it would facilitate home working, with a local "hub" for their office days, meetings, etc.