Bristol and the wider area is having a good bit of rail growth (and indeed the economy and station precinct is doing well there) -
in the future there could be an additional:
1tph Oxford - all week
1tph Henbury (could see this going to 2tph and/or looping via Avonmouth in time)
1tph Portishead (this might also grow to 2tph)
1tph additional Cardiff service slated
1tph Birmingham Moor St (part of earlier Midland Connect scheme)
1tph XC extended to Exeter (long discussed) - or something else in that direction, esp when Wellington/Cullumpton open
Also I think there's a WECA Plan for an additional 1tph on the Severn Beach Line (to give 3tph to Avonmouth), and TfW plans to have local services between Cardiff and Bristol for their local stations recommended in the Burns Report.
Worth noting that of the services listed, only the Oxford, Henbury and Portishead are committed to firmly. Bristol-Cardiff is being served by a committed 0.5tph between Bristol and west of Carmarthen.
if all of this happened, plus anything else - those platforms might be very useful.
The problem with any new platforms in the 'old' shed is that you lose platform 1 to provide access, and they're all on the relief line side, so unless you dedicate those platforms to services to/from the Filton Reliefs terminating at Bristol then you just add in more conflicts to a very congested junction.
To take your list:
Oxford - slated to operate via Bath, would terminate on the high-numbered sidee.
Henbury - could use the new platforms, but highly likely either it of the existing Filton train would work through to Portishead.
Portishead - wouldn't use the new platforms as on wrong side of station
Cardiffs - TfW's trains would be able to use the new platforms, although it's possible these trains may need to use the Filton Mains to get around local trains (especially the West Wales, the locals would probably use the Filton reliefs.
Birmingham - would come in on the Filton mains
XC - running through so can't use the platforms.
They're a bright idea, but sadly unless people are willing to break cross-Bristol TM connectivity in the local services (or provide more capacity for 'right handed' moves to/from the Filton Mains) then they'd be a beige elephant.
but this thread is about the works to the train shed on platforms 3-6, so perhaps a new speculative thread would be the place to continue the conversation?