The OP was saying start and end some there
Which because border controls need staffing is only going to work if there is an hourly service, it wouldn't be cost effective for an occasional service (hence why Marne-le-Vallee was dropped). So that means you have to pick one from Stratford, Ebbsfleet or Ashford as your second terminus, and then you need to get decent loadings on those hourly services. I agree there is a market for those driving or living south or south east of London, but I doubt its a full train per hour.
For a lot St Pancras is a convenient starting point, so to get those people to swap you have to offer a cheaper option, and find a price point where the less convenient service is attractive to people. I think Ashford would probably make most sense, but could current South Eastern high speed services cope with say half a Eurostars worth of passengers and luggage per hour?
And seeing this is a speculative thread, in a parallel universe if HS1 and HS2 had been properly linked, to allow through services I could see an hourly (Somewhere north of Birmingham?) - Birmingham - OOC - Stratford - Ebbsfleet - Ashford high speed connecting service being a possibility, as this would help to ease the load at St Pancras. Offering through ticketing for international passengers. Timings could have been arranged to allow through passengers to be processed and loaded onto cross channel services without too much of a wait. BUT.. that bird has well and truly flown, however I think future generations will be highly critical of the way the two are lines are so close, but not linked, and at some point way in the future a very expensive project to link the two will be undertaken.
Its also worth looking at this from a Eurostar point of view, they have a popular service, for which they can charge top dollar, why divert resources away from this. Its really up to a competitor to come in and try carve a niche, as well as rail competition maybe there is a market for flights without the downsides of current low cost operators, with a higher initial ticket cost (but cheaper than Eurostar) and without all the catches and 'upgrades' that the current low cost operators then use to make the operation profitable.