Currently there's a flood of new megaprims being released, but they can only be created by hacking the viewer since apparently the server supports them, but the viewer builder and LSL throttles sizes at 10M.
This is not a good solution. All the problems of megaprims will exist, but a lot of the benefits are lost. There already are lots in use, and with the new unofficial tools there are going to be lots more inworld. If SL can support them, then make it possible to build with them, if it can't then ban them.
If there need to be restrictions such as they can't overlap a region or parcel boundary, they can only be phantom, not larger they X by Y by Z, whatever, please enunciate the rules and enforce them on the server.
New Feature
Open
Normal
Either support megaprims or don't
The new megaprims behave better, since they don't rely on prim torture to be the size that they were created at.
But LL still needs to do a lot of work to properly "liberate" megaprims.
They don't show correctly at all on the mini-map, and generally blot out a large area around them on that map. (See VWR-7207 for a feature request to fix that)
According to the office hours discussions, they are working on tools that will allow removal of an encroaching megaprim even if its roor or center is not on your land. That will eliminate the worst griefing use of them. They are also working on a sim-level method for exempting certain specific megaprims from removal, so the sim owner can have a feature like a sky dome or a lake surface, and can prevent its removal.
Making sure they can't be physical is a good idea.
Forcing them all to be phantom is not necessary, now that Havok 4 handles hollow megaprims much better. And we need non-phantom megaprims for things like floors in sky platforms.