Older thread but as others probably looking at it, we kinda of know that there will be sharing of content, custom is un-exactly sure atm, as when it's been brought up the devs did allot of dancing around the question by saying " we will have a ton more content and constantly adding content while players will be able to share their content within a community built in content sharing system" not an exact quote but we only know there is some kind of sharing, I think custom they are thinking "sure you can make buildings, layouts in our engine and then upload those to share with others", in other words you won't be able to use a 3d coder or make things not available in the game, but you will be able to share things like land molding and buildings you put together in game even full out theme parks ?
Meh I get why they are doing this and in a way I agree. The reason that RCT 3 customized content took off was not only the talent of the user base, but that RCT3 had no more additional updates after the last expansion. So it was up to the community to make this happen. Sounds like from my perspective that Planet Coaster developers are looking at the long haul of "years" of constant development and additions too keep the game alive as long as people are willing to pay.
The differance, there will be a cost, but at least we will know that content won't crash, hack or kill our computer

Most of all RCT3 stuff was pretty legit, but some wasn't. For myself however, I prefer a more direct content from the actual developer knowing I will get the quailtiy and follow up for that content , while it was nice to download "free content", it also came with lack of quality, polish, headacks and many times sites that would cost me one way or another for just trying to download the stuff.
Meh I get the want for custom content sense the community as a whole helped make RTC3 last so long, I also helped make content for such, but it wasn't the greatest way of making it happen that's for sure.