By far, Portishead. I can't think of a cause more worthy than that around Bristol. Trains between Portishead and Bristol Temple Meads perhaps with extensions towards Severn Beach. Sort of like a 'Severn Explorer' service

Reinstatement of passenger trains between St Andrews Road and Bristol Parkway with a new station at Henbury. With the 4th platform soon to open I can't see many problems with capacity at Bristol Parkway, and this would provide many more travel oppertunities with people able to get to the inner Bristol stations from Cardiff and the Midlands. Not to mention Henbury station would provide easy access to Cribb's Causeway.
Turnback proposal at Clifton Down may be interesting and something to look into, though I don't want a resulting cut in Avonmouth/Severn Beach trains due to that. And this sounds a bit silly but I was seriously wondering whether it's an idea to look into upgrading the line between Bath and Bristol , since it is one of the busiest in the region.
Everything John said too
