With regard to the RTA station it’s interesting Cleveland was one of the first cities with direct train service from a airport to the central city. However I think traffic from the airport is not great mostly employees. You could have a shuttle bus from the new terminal to the existing RTA station.
With regard to D concourse sitting vacant with no plan just United paying lease it was built where rental car lot was why not turn D into the rental car terminal.
Other airports due the same off site consolidated rental car facility PHX dies but I prefer to rent cars at the airport.