I would have made one of the first priorities sorting out pedestrian access between Cogan Station and the footbridge over the Ely into the so called Sports Village /Bay Pointe area. Already, we have the white water centre, swimming pool and the new ice rink in this area. Throw in the Penarth Marina area and that adds up to the potential of many journeys being made.
The area is shown here:>
https://www.google.co.uk/maps/@51.447768,-3.1847118,740m/data=!3m1!1e3!5m1!1e1
So, the local rail system in south Wales can deliver people to this location from a wide area - often using one train with no change. Unfortunately, they have to try and cross a very busy road in order to get to & from the station - which means that relatively few people do this. Unfortunately, the underpass under the main road was sealed up and a station on the Penarth line was closed. (This would have made a great Park & Ride but unfortunately the planners allowed a Tesco to be built on the site where a car park could have been for such a station. The present Cogan station does however have some parking).
I would have thought that the arch under the main road next to the Cardiff bound track could be used to fit a walkway from the platform - which could then rise up over the railway to link with the Port-Y-Werin footbridge. Thus, Cogan station could become Cogan - for Penarth Marina and Sport Village.
The relative cost v gain would be massive.
I also think that a shuttle bus say costing £1 return should link Cogan station with Llandough hospital. (A similar link should be made between Heath Halt and Heath Hospital with buses ideally timed for the less frequent trains on the City Line). Signs on the stations should advertise such bus links - should they ever happen. A quick gain for little outlay/risk.