There should be option to add rides from list to rides available in existing scenario. Now when you download some custom rides you need to edit whole scenario etc. You should be able just to add one of the wanted item to build window during the game (the same with scenery). There are also some scenarios in which you want to build certain ride but it is not avaliable, so it will be easier if you can just add them without editing whole scenario.
Another idea is possibility of purchasing land/rights-to-build even when scenario does not have any designed parts with option of buying. There are a lot of nice scenarios in which you want to play more but you ran out of land and avaliable rides
One more thing - are Six Flags scenarios savable with OpenRCT2? Because in original game this option is blocked. You can play Six Flags scenario but you can not save it.
