Well I am fine with sets being released for adults, but they should not alienate children. Modern armies is a definitive no go since many of these things will find their way to children's playmobil as well.
For adult sets, I would think more about sets a bit too big and maybe a little too complex for children. Such as a beautifull three master with soldiers on it, larger buildings and maybe playmobil depictions of famous buildings, moments in history, etc.
All kinds of things that would be beautifull to put on display, but wont alienate children.