I can understand the logic in calling at Morpeth and Alnmouth, but if there was a burning desire to have more services calling at the Chathill, Cramlington, Pegswood, and Widdrington, then there would be a requirement to do so written within the Northern franchise agreement. Given there isn't, it is safe to say that there is no necessary reason or requirement for ScotRail to serve them should they ever be granted the paths to serve Newcastle.
Cramlington is actually larger than Morpeth is, however Morpeth gets more calls as it's the county town, and Alnmouth, which is a village smaller than Pegswood and Widdrington, only gets calls because it's used by people travelling to/from Alnwick. Cramlington has 10 well used buses to Newcastle per hour, which as they're operated by Arriva, Northern see no reason to compete with them as Arriva will be getting the revenue regardless which option passengers use. There is a demand for a more frequent local service on the ECML north of Newcastle.
Acklington and Chathill are both very small places unlikely to produce much demand, and Manors already has a very good Metro service into Newcastle, hence why I didn't include those three stations in my suggestion.
That's a fairly major part of the train.
The options would be:
1. Use 385s, with their normal home depot up at Millerhill and Craigentinny ideally suited for their operator ScotRail, and have occasional maintenance done at Heaton alongside other Hitachi products which will share technologies. Also, 385s are the trains currently proposed for any extra ScotRail services along the ECML.
2. Use 350s, which are completely alien to the ECML. After the 385s are introduced the 380s will be used almost entirely around Glasgow. Scotland-end maintenance would require a transfer over to Shields, while Newcastle-end maintenance depends on how closely related electric Desiros are to 185s. Even then Heaton isn't the main base for the 185 fleet.
Your original point was that 385s could be serviced overnight at Heaton, because 800s will also be serviced overnight there and both 385s and 800s will both be maintained at Craigentinny. The same could easily be argued for the 350s as they're maintained alongside 185s at Ardwick. The extent of maintenance carried out on 800s at Heaton will be no more than the extent of maintenance already carried out on 185s at Heaton.
There are actually two other options:
3. Use 365s, with their normal home depot of Millerhill (or is it Shields Road?) suited for ScotRail, and have occasional maintenance done at Heaton, alongside another BREL product (158s, although they probably don't have much in common with 365s). Considering the issues that have been had with the 385s, it'd be very surprising if any of the options were taken up unless Hitachi offered them for free, therefore I personally think 365s would be just as likely as 385s.
4. Use 158s, which are already based at Haymarket, ideal for an Edinburgh based service, and some will also be based at Heaton very soon, making both depots locations well suited to maintaining them. Only two issues with this option are 158s are slower than the other options, and ScotRail currently have a DMU shortage (though hopefully won't once the 385s are in service)