Obviously this would never work here in the UK and would certainly be illegal in some way. However out of curiosity I would like to know specifically what the legal implications would be, or what legalities would prevent me from doing so (such as 'she is not a possession of mine' as one example).
Are there any cultures where this sort of thing is legal or happening?
This is all theoretical as I don't have a girlfriend or a wife anyway!
If a line in your will bequeaths something that you don't have the power to give (e.g. you bequeath something that you don't own at the time of your death), that line has no legal effect. If I died and left you the house at 10 Downing Street in London, for example, you wouldn't actually be getting it.
If your will contains enough of those lines and/or they seem excessively unreasonable, it may cause the validity of the will to be challenged on the basis that you weren't competent to prepare and sign it.
If the people reading it think it's reasonable, it may have a social effect based on what it conveys to them, which could lead to voluntary compliance with your wish (especially if the main obstacle to that being realized is a mistaken understanding of and desire to respect your wishes). That could help make peace, for example, if a surviving parent's remarriage would otherwise be opposed by children (or the surviving partner) or others based solely on a mistaken understanding of the wishes of the deceased.
It could also make for a really awkward moment, depending on the views of and relationships between survivors.
Addressing user662852's comment on the question:
You can also use a will to name a guardian for anyone you have guardianship over, which is usually more important for children (e.g. see "Why Every Parent Needs a Will.").
