In regards to a possibility of Arrendale in the motorboat cruise area, this is what i think could happen without loosing any other attraction in that area.
So just for comparison lets say a proper Arrendale darkride used a good size showbuilding. Lets take IASW showbuilding and move it over to the motorboat/Autopia area of the park.
As you can see the IASW showbuilding, without modification, would fit nestled against the submarine cave entrance on the south and the original tomorrowland Autopia on the east.
I modified the original track for the original layout of the past tomorrowland Autopia and highlighted in orange. I also added an additional 180 circle that could be added in dead space. If they felt that Autopia was to short at the point then maybe get a bit creative and update the attractions ride system so that parts of the ride are self propelled while others are automated. They can even add restraints and then maybe figure out a way to elevate the track so that it merged with the existing peoplemover track that runs throughout the land. It could be an interesting experience of a multi level Autopia.
I would take it a step further and if they are really still considering adding Tron coaster then theme the Autopia to tron. The coaster would take over the innovention building and have the outer portion fly over the Auopia queue. To add to the theming then the section of Autopia that would be north of the Arrendale show building could be enclosed as well. i marked that area in violet. now how cool would it be to go inside a tunnel and have a recreation of the old Tron tunnel that existed in the old Peoplemover.
Now getting back to the picture i posted.
The yellow highlights would be the part of the monorail that could be eliminated to open up air space. The light green line would be the area where the modification would need to be done to connect the monorail track. At this point the monorail would no longer have the "8" figure track portion and would just go around the outer edges of the expansion pad.
The brown buildings with black highlights could be part of the Arrendale village which would hide the show building and also part of the monorail pass on the northern edge.
The brown rectangle with the green outline is the old and still existing motorboat cruise queue section. This piece could stay but be rethemed so that the bottom section and roof covering matched the aesthetic of Arrendale. I would go a step forward and maybe even add a themed wooden bridge like covering on top of it so that the monorail goes inside of it. This is not necessary but if they have an issue with the monorail destroying the Arrendale theme then this would hide the monorail from being visible in that area. The monorail would not be visible until it starts heading over the Matterhorn pass.
seems like they have plenty of room there to add an Arrendale area especially since the building would be built to fit the proper space.
Any large show building would be themed to macth the side it is facing. The side facng tomorrowland can be more rockwork like submarines if they intend of keeping it and the side facing the new Arrendale would be buildings with a snow acpped mounatin range in the background. The ice Castle can be built on top of the building preferably on the north side to help with sitelines. no need to touch the matterhorn which would now have an expanded snow covered area to compliment it.