According to sources at WDWNT, internal documents at Walt Disney Imagineering are pointing towards a April/May 2019 opening.

Mickey and Minnie's Runaway Railway

While this has yet to be confirmed officially by Disney, it does make some sense given the focus in 2018 is likely to be Toy Story Land and the announcement that Star Wars: Galaxy’s Edge would be opening later in 2019 at Disney’s Hollywood Studios than its counterpart in Disneyland.

May 1st 2019 is also the 30th anniversary of Disney’s Hollywood Studios and if I worked for Disney a Mickey Mouse themed attraction certainly lends itself to opening on that date…

Personally I would think it better than Disney saves any opening for 2020 as everybody going to Walt Disney World in 2019 is likely going to want to visit Star Wars though perhaps another attraction will take a small segment of the crowds away.