Isn't Frontier working on a community share program that will allow us to share our custom builds and parks with others to upload into their game? Couldn't the same process of multi-player work that way, since a lot of builders are good friends to each other? Say, you speak to a couple of people who want to build a group park project. Instead of multi-player, that group talk about who is going to work on what. So person #1 works on the front park entrance, saves, and shares the park and what they created to person #2, then they work on the 1st themed area, share, then person #3 does the 2nd themed area. The process repeats itself until the park is completely finished and then everyone who worked on the group project took credit for what they contributed.
I personally am not a fan of multi-player, but just because I don't like it doesn't mean it shouldn't be in the game... I just would never use that feature. Played multi player with Simcity 2013 and Minecraft and people would just either alter something I worked hard on, or completely destroy it and type "HaHa". It's not cute or funny when you put a lot of hard work into creating something just for someone else to alter it or destroy it.