Integration of X7123M3-256's custom coasters
Obviously, this would require communication between the OpenRCT2 developers and X7123etc...
Addition of track sprites not shared between coasters
This is one of the issues with the custom coasters mentioned above. The coasters that should have vertical slopes and inversions simply don't, and the stock coasters (especially the wooden coasters) could really benefit from those smooth steep-to-horizontal transitions. Corkscrew coasters could also use banked sloped curves and the like.
Addition of new track sprites altogether
In the "Wild" expansion of RCT3, the "extended coaster" had access to things like diving loops, larger corkscrews, diagonal inversions, etc.. These would be awesome in OpenRCT2.
Catwalks and custom supports
Special scenery items that can merge with coaster tracks, allowing extra touches of realism for coasters.