The easiest solution in lieu of more 170s is simply to extend the Leicester stoppers to Stansted thus spreading the load equally and therefore least giving people the chance of a seat especially these who travel pass Leicester.
Even if these stoppers only got extended to Peterborough as you now have platforms 6 and 7, it should be possible to extend the service there especially as Birmingham only has a 10 to 15 minute turnaround so no reason why Peterborough can't do it especially as they already have a catering base at Peterborough.
You'd still need extra 170s to extend the Leicester stoppers: It's only a 25/27 minute turnaround at Leicester, while even just Leicester - Peterborough is essentially an hour each way. That's going to require a minimum of two extra units for an hourly Leicester - Peterborough extension.
To extend them all the way to Stansted (A 5 hour round trip from Leicester, excluding turnaround) would require almost as many extra units as would otherwise be needed to allow all current XC 170 services to operate as 3 or 4-car.
WMR (West Midlands Railway) will lease/store 23 of the 170’s and so this may allow them to join and form a 4 car 170 (2 2-car 170) which would be an improvement to Birmingham to Stansted services.
Six or seven additional 3-car class 170s would be enough to allow all Crosscountry 170 services to operate as either 3 or 4-car trains. This could be a possibility for the next Crosscountry franchise even if Northern were to take their fill of 18 2-car units from the ex-WMT fleet (Which requires a centre car of a 3-car unit to be moved around).
An order for about 15 5-car class 802 units, would pretty much solve the problem. They would quite cheap and could be delivered relatively quickly, as a follow on order to current production.
Definitely (and sadly) a consideration for the next franchise, but while I appreciate the desire to keep lease and maintenance costs to a minimum by ordering the minimum number of new carriages, based on current diagrams 22 daily diagrams formed of 7-car class 802s (Other bi-mode alternatives may be available) would allow:
All North East - South West services to be formed as such,
The HSTs to be withdrawn from Crosscountry service
A significant boost in capacity on this regularly overcrowded corridor
All Crosscountry inter-city services to be formed of a minimum of 5 carriages (Diagrams become either 7-car Class 802, 5-car class 221, or 8-car pairs of 220s)
I feel sure that agreement could be reached with Hitachi to allow maintenance of the trains to take place at existing IET depots conveniently located along the line of route at Craigentinny (Edinburgh), Doncaster and Stoke Gifford (Bristol), with stabling in addition at Laira (Plymouth).