Northbound, for example, you'd need to swap round the order in which the Nottingham and Sheffield services left St. Pancras as they are only three minutes apart, but if you did that with the typical xx32/xx35 services and each of them picked up each other's stops you'd be using the same two paths all the way to Trent South. After that, the Sheffield service would only be three minutes later into Derby, and I can't see any obvious conflicts there. Maybe the path into Sheffield from Dore is the issue? Platform occupation isn't an issue since it would just sit at Sheffield for 15 rather than 18 minutes.
Or - maybe more likely the issue is that to keep close to existing paths the Sheffield service would lose its Loughborough and East Midlands Parkway stops (the xx02 doesn't stop there so that would leave none). I can see that might be considered unacceptable - no service at all from Derby to Loughborough for example. And keeping those stops *would* seriously mess with the timetable.
I'm pretty sure there are good reasons why the service is as it is (although there might be disagreements about the definition of "good"!).
The concept of a Kettering call was assessed in great detail as part of the MML 6tph LDHS.
Ultimately, the core timetable we have now is the one that generates the most new passenger journeys, delivers the greatest commercial return and is, in my view, therefore the most optimum to the railway and wider society at large.
Any timetable is a trade off ultimately. Add a Kettering call in one of the Sheffields, knacker your slot at Dore and you’ve now got 1 effective tph to Sheffield.
Both Sheffield and Nottingham are similar size markets but with very different characteristics - Sheffield with years of incremental business and point to point growth is sensitive to journey time, and gains are easily lost if you slow trains down. Nottingham has a much broader customer base, and gains are far more easily made increasing connectivity to the wider region.
Put that together, plus power constraints at Corby before KO1a online, and you have yourself the timetable you see today.
When the Auroras enter service, will they have better acceleration and braking than the Meridians, potentially allowing an additional call to be slotted in without making a mess of everything?
No. It’s different but not helpfully so. As BR alludes above, there are some areas where 30s could be shaved off, but core timetable interactions prevent those incremental savings being banked (ie other trains get in the way).
Notably there are places where the 810s will be deficient, such as the Leicester - Trent corridor on all stops service under diesel mode compared to a 222. That can largely be accommodated though with 30s nabbed here and there to make it still work.