Is that even realistic, yet alone needed? The current 2 tph service (at least as far as Avonmouth) is the best service that the line has had for a long time and is probably adequate, as Bristol Temple Meads is very poorly sited for attracting passengers travelling to the city centre from suburban stations on the line. The only way that local rail-based transport could make a major inroad into transport modal share within Bristol, as distinct from tinkering around the edges, would be to construct an underground metro system, and the cost of such a scheme would be completely unaffordable for the UK. As for light rail, there really aren't suitable routes/alignments - the former Bristol tramway system was quite archaic, in terms of layout yet alone the open-top cars, by the time it closed due to enemy bombing damage in WW2.