Should something connect to this unused port on the coolant overflow tank:


Also my water pump is leaking a bit. When I replace it, are there other things I should replace at that time?

 

That is a coolant reservoir.
The hose/port is connected to the overflow tank (atmospheric catchment tank) at the rear part of the reservoir.
The 'air-bleed' pipe is the one nearer the front part of the reservoir.
The one NOT CONNECTED is 'blanked-off'.

 

Also, avoid using jubilee clips or worm drive clamps on hoses with connections to plastic components as the plastic will fail causing a sudden leak. Use constant tension clamps on all hose connections to plastic.