Just a few ideas they could consider

Reduce or get rid of the ridiculously long filter lanes which only encourage bad driving with people cutting back in.
Put average speed cameras in covering before the Coldra to after Junction 28, set at 60.
Reduce the number of junctions
Encourage more use of the southern distributor
Make the existing motorway 3 lanes as much as possible (dont know if a widening of the Brynglas tunnels is at all possible?)
Install train stations at Celtic Springs and near the Stats office