Standards like SMART-on-FHIR [1] and CDS Hooks [2] have the potential to allow innovation developed outside of Epic and Cerner inside of those products. Both of those vendors even have "App Stores" [3] [4]. So far, though, there aren't a lot of apps in these stores, and none of my doctors (all of whom work for large academic medical centers and use Epic) have access to any third-party apps - so I do wonder whether the vendors (or their customers) may be putting up roadblocks that are slowing adoption.

[1] http://docs.smarthealthit.org/ [2] https://cds-hooks.org/ [3] https://apporchard.epic.com/ [4] https://code.cerner.com/apps