Hi Naoko,
Thank you for your idea to help improve SDL TMS. For unknown segments, I believe this will be handled differently by allowing content across all files to be edited simultaneously (virtual merge) and then filtering out based on, for example, match percentage.
FOU functionality is something that is more interesting as it provides the opportunity to get a performance benefit. Having said that, future ways of working will most likely make this redundant as "real time" TM update and lookup will provide similar functionality with the added benefit of fuzzy matches and segment repair (upLift).
I'll leave the idea under review for now for others to comment.
David