For me, Frontier’s Object Database was the most compelling of all the things in Frontier. I never really go into the language or the outliner parts of his work, but the Object Database was clearly a good idea. So much so that I’ve been trying to figure out ways of incorporating it into AppleScript ever since. Products like ScriptBase tried, but none of us have been able to achieve the integration that Dave did.
The closest I came was with FaceSpan 5 where the entire UI and application were entries in a hierarchical object database, and event handling flowed up through the object tree from the source (e.g. a button) to the root (the application). Everything was mutable and persistent which made for a hugely powerful tool, much as Frontier had demonstrated many years before. Sadly, FaceSpan 5 was never finished.