Currently the only regular Avanti services north of Manchester are the Glasgow Central 'fasts' and the Glasgow/Edinburgh via Birmingham service. The latter is much slower for anyone wanting to travel north of Crewe - from MKC you're better off taking LNWR to Crewe via the Trent Valley and changing there.
MKC is busy enough to justify long-distance services to Preston, Wigan, Lancaster etc. - even more post-EWR. So while the only regular Avanti service to these stations is the Euston - Glasgow Central, it justifies a stop, even if only at peak times.
If Avanti decide to introduce a second hourly service on the WCML which doesn't take the slow route via the West Midlands, such as an additional Euston - Preston service, it would make sense to have it call at more intermediate stations to improve connectivity to the north, including MKC, Tamworth and Crewe.
Of course the answer is right in front of us: post-HS2, there'll be plenty of capacity for most trains taking the 'classic' WCML route to stop at Milton Keynes, this'll just depend on how ambitious a post-HS2 WCML timetable will be, and how much of HS2 is completed in the end. SNCF do still run 'Classique' intercity services as a low-cost alternative to high-speed rail. Something similar on the WCML would solve the connectivity issue to cities bypassed by HS2 and make long-distance travel by train remain affordable (as I'm certain HS2 will be more of a premium option, without offers like the Superfare).