I believe assemblies/conventions will continue to be shorter in duration as the W/T is running out of things to say and witnesses are tuning-out that which they've already heard 100 times over.
There had been a prior post where it was mentioned that DC's would soon be held at Assembly Halls. I recall that it was over 1 year ago. I searched but cannot find that thread.
Perhaps the SAD will be discontinued?? (speculation)
I too think that the mid-week days might be utilized as DCs had previously been held through the week. As the DCs are generally scheduled during summer months when most people schedule their vacations and children are on school break, W/T would expect JWs to schedule their vacation time when the DC is being held.
I think another component of utilizing Assembly Halls is the ability to accept digital payments through Debit/Credit machines. Debit/Credit payments may be working so well that income brought in through digital access may be greater than the old cash method. I doubt that implimenting this means of payment is easy within a rented venue, especially when utilizing numerous machines which are decentralized throughout the entire facility. One alternative is cellular data access but this is an additional cost and signal coverage may be probelmatic depending on the venue. If the digital payment machines are already set-up at the assembly halls as I understand they now are, they could be utilized more often at little additional cost.