For starters, I don't think there'll be tens of thousands allowed in any of the parks, period. Attendance in each park will be measured in thousands. Can Disney make it work financially on thousands? Not at full capacity, which is why I think we're seeing talks of entertainment cut backs, ride cut backs, cut operating hours, staffing cut backs etc... If they drastically cut into the experience, I would think that they could probably make it work with say, 10,000 per park, when you add in the money they'll make off merchandise, food etc... 10,000 sounds like a huge number, but when you spread those people out throughout the entire MK on rides, in shops, restaurants etc... I don't think it's that unfeasible.
Mickey head stickers on the floor in all queues (rides and restaurants, entrances etc...) to space people out. Virtual queues. No entertainment (parades, fireworks etc...). Theatre-style seating attractions run at limited capacity with each person/family seated 6' apart, every other row. Restaurants are very easy to set up to maintain distance. Maybe even staggered park entrance - let so many in from 9-3 and then again from 3-9. Doubles their daily totals while maintaining necessary limits.