I considered a new indoor coaster across the path from Mummy, but I think that’s not what the LL needs right now. It’s also somewhat small, and I wouldn’t want to box-in the Lower Lot (just in case they go to expand in the future toward the front of the Studio). Plus, it would balance out new rides (JW>Globe>Existing Tram Space).
As for the preview theater, the idea that it could be built for trailers, but possibly hold a full-blown live show one day. That would be a nice get. Maybe it could be big enough for Grinchmas? (Admittedly, I’m only familiar with the Orlando version).
If Jurassic World is 2019, and a SLOP/Globe replacement is on the way for 2020, a relocated Tram Tour (new host segments/tour animation? Maybe Mortal Engines if that’s a hit.... swallowed by a city) and Monster Ride/Walk-thru additions are a nice fit for 2021. Then you’d have Nintendo and a new indoor Coaster to look forward to in the future (plus the Teacups/expanded NEST Base, which would probably go into the space on the right of Transformers).