As to Cross Country, I just want to see the existing Leicester terminators extended to Cambridge with option of being extended to Stansted so it's a half hourly between Leicester and Stansted instead of a hourly service which can be very busy.
That and just run 3 car DMUs on the route and displace the 2 car 170s elsewhere as 2 car DMUs aren't good for a Inter Regional Airport Service.
To increase the all the Birmingham to Leicester/Stansted routes to run to Stansted would require 15 units, which would mean if the Cardiff services were all EMU's then there would be enough (with two spares) to run all of them with 3 coach units (as they have 17 3 coach 170's) on a half hour frequancey. If XC also retained 6 of their 2 coach sets to allow all the trains to run as 5 coach trains between Birmingham and Leicester (where the largest passenger loadings are likely to be, given the current timetable) and then run as 3 coach trains onto Stansted, that would still free up 6 two coach sets to be used by anouther TOC.*
* Although I'm a bit confused (so I'm not sure I've worked out the number of units required correctly), as looking at the timetables it looks like that (with no spares and no doubling up) XC only requires 7 class 170's for the Cardiff Route and 10 for the Leicester/Stansted routes. However they have 29 units, meaning that there are either a lot of doubled up units and/or a lot of spares and/or they are used on routes which Voyagers should run to allow more Voyagers to be run doubled up.