Chippenham is not so much an intercity hub, but more the only station serving a lot of towns, upto about 10-15 miles away. However recent road changes and lots of additional traffic lights have made it a painful location to drive to.
If there was going to be a hourly stopping service towards Bristol, would be better to reopen the north curve at Bradford junction and go via Melksham. Although if Corsham station reopens then current routing would be needed.
Longer term Bristol trains would be better to skip Didcot and Reading, but add Old Oak (whatever the new station is called), but Temple Meads is a poor destination, should probably carry on to Weston super Mare (at least every 2 hours).
The area around Keynsham has expanded and road access from west Bristol is much easier to Keynsham than Temple Meads. Similarly it is easier to drive to Corsham from Melksham, Trowbridge etc than get to Chippenham station.
So ideally want a good semi-fast service Bristol-Swindon, with cross platform interchange into fast trains to London. Would make more sense to continue the semi-fasts to Oxford and Bletchley than going to London. Yes the ideal train would be a regional bi-mode (same as Cardiff-Portsmouth needs).
I don't think Bristol TM really needs additional fast trains to London, perhaps longer 11car versions that stop more west of Swindon, and skip Didcot and Reading (those sort of places have enough other trains). Same principle as a West Midlands train running non-stop Euston-Coventry, missing Milton Keynes.
Realistically have to look at serving Bristol area, not just the bit south of river where Temple Meads is located. I suspect not many people want Paddington either as a destination, but continue to West End or City. That end is covered by Elizabeth line. But at Bristol end if got to connect to get to final Bristol destination in its suburbs might be better with the train serving the suburbs connecting with cross platform into the fast. I think it might even be possible to do this with some creative timetabling at Chippenham by using both sides of the island and the existing bi-directional signalling, especially if only connecting hourly in each direction.
Having lived in Corsham and now Bradford-On-Avon hope this local knowledge of Road system and station location helps.