I don't know much about the Vistana or Hyatt current systems, but am familiar with the Marriott enrolled DC points program which we use (we didn't buy any trust points beyond that).
So my speculation is just based upon what would work for me, not upon what may be financially feasible for Marriott.
First, I will not buy new trust style points at their high rates to gain access to Vistana or Hyatt. Those are just icing on the cake to me, and I can take them or leave them, though getting them occasionally through II trades (usually off season) is usually enough for us.
I would prefer a new system that allows me as a Marriott points owner to have a preferred access of a sort to Vistana and Hyatt, but not as full of access as points owners of those systems. Something similar to how Worldmark-only credit owners get access to Wyndham at 10 months out, rather than at 13 months out or whatever if you're a Wyndham owner. I'd still have the option to get full access to Vistana and Hyatt by buying their points at the full rate, but could get this more limited access to them by just being a Marriott points owner.
I would expect the same of Vistana and Hyatt owners in their ability to access each others' current properties, plus Marriotts. No full access without buying DC style points at the full rate for each particular system, but this sort of 10 month out limited preferred access instead.
For that, I'd be willing to pay a small annual fee on top of my current DC fee of $270. So for example, if I want the limited but still preferred access to only Vistana or only Hyatt in addition, I might pay $320 per year, instead of $270. If I wanted limited but still preferred access to both Vistana and Hyatt as a Marriott-only DC points or trust points owner, I might pay for example $350 per year instead of $270 or $320.
I think it is important that they not open up our existing properties for full access by owners of the other 2 systems without a buy-in that substantially asks you to become a regular points owner of the other systems. I do not want to see my Marriotts all of a sudden becoming very hard to get due to owners of the other systems because they don't have to pay much or anything to gain this access.
But limiting the access to the other systems to a 10 month or so window for a reasonable fee will allow existing owners of each system to grab the prime weeks in the 13-10 month window ahead of time for their own systems, and still provides lots of options for other system's owners within these 3 systems to get better units at 10 months out than any non-owners of any of these systems could access through trades in II, etc.
In terms of weeks trades through II, I also think that the Marriott preference, Vistana preference, and Hyatt preference should still only accrue for the system(s) that you already own for the current owners, and to new purchasers of each system, and not be extended automatically to owners of the other pre-existing systems.
You'd thus need to buy units or points for each system to get this preference, not just have it extended to you because as a prior Vistana or Hyatt owner you're now a part of the new larger Marriott system, and the same for current owners of Marriotts not getting preferred II trading into Vistana and Hyatt just because they merged.
How that would change if Marriott decides to ultimately fold all current systems into one big megasystem and blur the lines of which one you own is a complication to this approach though.