I think Sywell is worth another look, despite GeeRam's well made earlier comments. It has many advantages including parallel hard and grass runways, although each are only about 80% of the length of Duxford's, and it is only around 40 miles/15 minutes flying time from Duxford to allow pilots to shuttle backwards and forwards and swop aircraft. Sywell also has an active war-bird community including restoration workshops, and the airfield has very much the right "ethos" in regard to vintage aviation with for example its art-deco era clubhouse. Sywell is also located in reasonably clear airspace away from both Luton and Stansted movements.
The logistics and costs of car-parking for say 20,000 visitors, together with all the catering and toilet etc amenities required to be brought in will be more-or-less common where-ever it relocates to and hence will probably not be a critical factor in the decision. It will of curse be a critical factor in the budget.
I was Chairman of the organising committee of a large sporting event for ten years. We relocated the event at short notice (5 weeks) due to foot-and-mouth disease closing off our usual outdoor venue, to a completely new venue. Not only did we have a very short time to make the move, the move unexpectedly resulted in a three-fold increase from around 2000 to 6000 attendees. Therefore I don't underestimate how big and daunting the task is. Small beer compared to FL perhaps, but what I would say is that where there's a will, there's a way.