The problem with the monorail (in its current form) is that it's essentially a
Disneyland ride. When you enter the DTD monorail station, you're entering Disneyland. That's why they didn't build a DCA station when it opened.
If the monorail goes elsewhere, they won't be able to check tickets at the DTD station. They would have to check Disneyland guests at the Tomorrowland station. That's a massive logistics and security headache by itself. There isn't a lot of room around the Tomorrowland station to hold people while they process tickets. (Possibly at the exit next to Nemo, but it'd still be pretty dicey.) Bigger problem: if, for example, a group of people doesn't have tickets and can't actually enter the park - they're technically inside the park already. Now, you've got to push them around to the monorail entrance somehow and back onto the monorail to get them out of the park. Even then, you're putting them in a line with guests who
do have tickets and are already in the park. It becomes a maze.
Broadly speaking, it's better to check tickets before they get on - not only for security reasons, but it also staggers the guests over the time between monorail arrivals. Doing it in Tomorrowland would mean potentially checking an entire load of monorail riders at the same time.
To make it work, they'd almost certainly have to lose the Tomorrowland station. You could still ride through the park, but the most-likely fix would be building a new Esplanade station outside the park, possibly on Harbor behind Tomorrowland (after you leave Disneyland) over the local bus stops. That would serve both DL and DCA. Then, if they decide to do something at the Toy Story Lot, they could then run the track down Harbor. However, the new TSL station would have to be behind security (since the other two stations already would be). (There are also some right-of-way issues as far as building a TSL route.)
The other trouble there - if the monorail goes more places, you'd have more people trying to use it -
a lot more. The current monorail tech (and capacity) is designed for the small subset of guests coming from the west side of the resort going into DL. It's also very manual - not a lot of automation. If everyone can use it - it would need to be able to handle everyone going around the resort. It would basically have to be more like an airport train - potentially full automation, running on a regular schedule.
(There's a quieter question, too, about whether the track inside DL meets modern transportation standards - can they upgrade it without having to rip some of the tighter sections out, or removing the DL stretch altogether.)
It's not impossible, but there's a point that it wouldn't be "the monorail" anymore. It might not be bad - but it'd be pretty significantly different. (And it'd be expensive.)
