Question:
Which services will stop at Slough?
As now, the Oxford and North Cotswold Line trains will stop at Slough - presumably omitting it during the peaks as they do now.
Would agree here, the 00/15/30/45 departures for Bristol out of Paddington would encourage non-savvy passengers to take the slower via-Bath services (not realising they are overtaken), while having 15/18/45/48 (Parkway/Bath/Parkway/Bath) would make it clear that you have two classes of Bristol services, the first designed for Bristol passengers and the second designed for Reading/Swindon/Chippenham/Bath passengers (both from London and towards Bristol), the latter leaving straight after the first.
Many different options could be available, but of course it's not quite as simple as just altering the times from Paddington as that then affects the dwell time incoming units have at Paddington, as well as the pathing of them further down the line. You then need to consider patterns and how that might then affect other services to provide (for example) Didcot with a sensible spread of departure times from Paddington as well as a train per hour to Bristol and a train per hour to South Wales - ideally the Cardiff one, so the Swansea journey times can be reduced by the maximum amount.
Then there's the time they present themselves at Bristol, platform availability there and occupation time, as it stands there will typically be two 19 minutes, one 23 and one 30 minute turnround at Bristol each hour, which is probably just about spot on in terms of providing enough slack for a bit of late running, and minimising platform occupation. Then you need to consider rostering the crew giving suitable break time for staff at Paddington and Bristol, but also maximising their efficiency.
Also, advertising four trains to Bristol per hour at even intervals is good for marketing.
So, to sum up, lots of other things to consider!