In case that there is no option to define auto assignment, we recommend activating the smart recommendation feature on relevant roles, so the manual assignment will consider users that have been involved in similar ideas in the past, and therefore more likely will be good candidates.
This can easily be enabled per role via the subsystem setting-->workflow--> role setting:
And per feature owner (e.g. experts, task owners) from the subsystem setting-->workflow--> features --> setting of the specific feature. 