The best plan would be for the City to take over the EWEB headquarters, converting the north wing into city council chambers, and using that move to inspire further re-development of the rest of the EWEB property.  

They could build an addition to the south of the EWEB building - allowing them to house all of Central Services in that one location. 

That would allow the county to occupy all of the former city hall site - erecting a new courthouse with adequate parking.  

There is no operational need for the remainder of City functions to "return to City Hall".  

The now demolished city hall building was planned to be the home to nearly all city departments - and nearly all city departments were originally housed there - each with a sign hanging in the courtyard.

But cities grow - and they out grow even the best planned buildings.  That will be true of the next city hall just as it was for its predecessors.