Over the years we've seen how talented and skilled people have continuously added new content to the previous games to help keep them alive. Without custom scenery and rides, the vast communities that formed around these games would be a lot less active than they are now.
The idea is simple, allow the community to give a helping hand in the game. Valve has probably been the best example of a company who've directly reached out to a community for new items and maps. Because of this, games Like TF2 have gone from being a secondary title to one of the most popular games on steam, with regular updates and items created by the community.

RCT can also benefit from its dedicated community. Provide creators a resource pack (with all the necessary information relating to file formats, scale and size etc.) and encourage them to make new content for the game. Allow the normal steam workshop activities (like subscriptions to individual mods) but also use it as a source for new content that can be added to the core game through updates. I deeply believe this will increase the longevity of the game and keep players interested.
Many people have mentioned mod support and steam workshop before but (as far as I know) nobody has considered taking the valve approach to community created content.