[schapplm]

way to make a user friendly "To Watch"-Watchlist[...]Is that something that crossed your mind as additional functionality or separate addon? (Not an easy task and probably too time consuming, just wondering)

I also have thought about this issue from time to time.
Creating a database with unique identifiers from within Kodi (e.g. imdb number) would require scraping the media somehow or obtaining the identifiers on other ways, so this would mostly be suitable for media that is already in the database.
There are already a few solutions for this. E.g., the trakt addon or lists in streaming addons like in the Netflix addon. There is also a "WatchList" addon, but I am not sure what it does.
I think you should start a discussion thread on this where you specify your requirements and compare them against existing solutions.