As long as we can try and ransom any character, for any reason. I hate it When my grandson is imprisoned and I cannot do anything to try and get him out
I think that ransoming them should be possible - but only when you know who stole your grandson of course or when they approach you. But in general I would say that the chances to steal your grandson, given he resides in a proper court, should be low and mainly enabled by having enough enemies/amoral courtiers with influence in court.
Much easier option would be stealing lowborn people, lesser nobles or, mainly, society-related plots (for example, kidnapping a maiden to interrogate her about Paulicians with a support of your holy people in court).
But in short, I think that it should be hard to do at first - and it can be adjusted later.
Could be related to plots, too. Imagine kidnapping someone you suspect is part of a plot, so he can spit out who are his collaborators
I think that this indeed can be a good alternative to imprisonment, especially if it seems quite risky. Though, unlike imprisonment, it would be harder to do - but if you have some leverage on them (a lover maid, some gold, traitors) you could try to pull it off.
It would be a more satisfying alternative to "just" killing them. But I do hope that it won't be as unclear as current abduction in CK2.