Personally I'd be in favour of a group. I very often purchase tickets from Bletchley to places north on the WCML which forbid break of journey on the outward (or at all if an Advance). Unfortunately however the connections from Bletchley to MKC can be a bit unpunctual, and for some destinations they are rather tight (or if you add in extra change time far too loose). As such if it looks like the train up from Euston is starting to pick up delay I prefer to go direct to MKC in a taxi instead of walking to Bletchley, as I generally prefer getting to my destination on time over being an hour late and getting Delay Repay, particularly if travel is in the evening (when this is the greatest problem).
Obviously it'd be possible to refund the original against a £5 fee and repurchase, but that's a faff and means going back to Bletchley on the way back costs more. And I don't see why I should pay nearly £10 extra on top of the taxi fare I'm already paying because the railway can't be bothered to sort its punctuality problem out.
It happens to be the case that for most destinations I've tried (I suspect not Euston) the gateline at MKC does accept tickets with an origin of Bletchley, and staff are also OK with this despite the break of journey restriction officially forbidding it. But given how hardline the railway can be at times (I wouldn't put it past them at some time in future to start looking for evidence of starting short on tickets with break of journey restriction and sending nasty letters demanding settlements, for instance) I would prefer it to be an official Milton Keynes Group, and if it was then this would be permitted on walk-up tickets at least because the group would be the origin.
For what it's worth, though, if buying tickets that allow break of journey you might as well just buy the longest as the fares are generally the same, e.g. if getting an MKC-Euston season just get a Wolverton-Euston one, it costs the same and you get the extra validity free in case you might want it once or twice.
The one exception is Bletchley-Euston, where the fares are different - the Anytime and season is traditionally slightly cheaper than MKC and the rest of the structure is totally different due to Oval. This could I suppose be sorted out when Oval goes to MKC, and put the group in then, or you could continue having slightly cheaper Anytime Day Singles and seasons from Bletchley-Euston specifically (noting the below, routed "Not Milton Keynes Central", perhaps) if you wanted to encourage use of Bletchley (e.g. because the car park doesn't get full).
Furthermore, it would be useful if there were suitable easements such that doubling back was always permitted within the group, as at some times of day an Avanti to MKC then a WMT back can be quicker than a direct Bletchley train (and the same for Wolverton for a journey to/from the north). You can obviously do that if you have a Euston-MKC season, but you can't do it on any single or return fare, and given that long distance fares are priced the same I think it would be good if it was allowed in this specific case (and similar ones).
I suppose this is a bit of an unusual case because station groups are normally just used (for fares) where there are a set of stations in the city centre, rather than spread across a city, but in this specific case I can see lots of advantages and few downsides. It also stands as an example of why break of journey restrictions on Advances (rather than simply a requirement to use the booked trains for any travel) are a bit nonsensical - and Northern is known to enforce against passengers when they do exactly the sensible thing I outline above.
