Assuming the future size and scope of the "traditional" Cross Country operations to stay broadly the same as the situation in 2025 (i.e. ignoring the ex-Central Trains routes/ Turbo stars), you're probably going to need sixty-something trains to replace the Voyagers (on the basis that each has more seats than at the moment, or runs in pairs more often, i.e. an upgrade but nothing ground groundbreaking)
Ideally one big fleet that could do anything a Voyager currently does
Certainly capable of being self powered (battery, diesel, duckweed/ whatever) and running off overhead electrification
However, of those sixty-something, only a tiny amount at any one time will be either (a) on Third Rail or (b) running at 125mph
How much should the tail wag the dog?
It takes three units to provide the hourly service from Basingstoke to Bournemouth , which includes a healthy layover at the south coast
And maybe half a dozen units north of York at any one time (the Aberdeen extensions take up many hours but are at the far ends of the each day when the train would just be sat at Craigentinny anyway), although little of the XC running north of York is 125mph - the line isn't particularly fast north of Newcastle -only one short bit of 125mph in Scotland - XC services often stop multiple times between York and Edinburgh - if we were talking 2035 then I'm sure there'll have been a recast or two on the ECML anyway
So, whilst the wishlists will no doubt include Third Rail and 125 mph running, both of those come at a cost, both financial and non financial (Third Rail means additional weight but more importantly making trains more complicated means more scope for things to go wrong - 125mph capabilities means sacrificing some of the space on board for "crumple zones" etc)
XC abandoned tilt because the benefits on short sections of the network didn't match the fleet -wide costs, so how much of the final design will be trying to introduce bespoke features that might only be used by 10% of trains at any one time (or even fewer)?
Or do you split the operation into a "limited to 110mph but can do Third Rail" for the Manchester - Bournemouth corridor and "125mph but no Third Rail, like an 810" for the Edinburgh - Plymouth corridor?
I've suggested doing versions of this before, operationally neater to simplify things, few people will be doing journeys like Doncaster to Reading on XC anyway - it'd allow two separate fleets