Unfortunately default values are stored in the designer code and thus would require a recompilation of the code to do (meaning you’d have to have UiPath’s source code - not doable without breaking license and some laws).
I’m wondering though if it would be possible to hook up to designer canvas events and inject a custom handler, essentially creating a plugin that would change the value after the activity has been inserted. There would be some issues to overcome (differentiating between anything that moves an activity [surround with TC, undo etc.] and actual addition comes to mind).
@badita - any chance of plugin support in unspecified future? Would probably need more use cases to be considered though.