I shudder at the thought of Streep as La Liz. She'd probably work up some sort of accent, pick one facial expression, and mug her way through the part. Can't see Kate Jackson in the role either, as much as I like her. If Faye Dunaway had allowed herself to age gracefully (have you see those Chicklet veneers?), then she might have been able to pull it off. Blythe Danner could be interesting. And suddenly I'm wondering what Shirley MacLaine would do with the role. An offbeat choice, but her persona might mesh well with the DS universe.
Perhaps the way to go would be to cast a British actress, such as (but not necessarily) Helen Mirren. I think a British actress would be able to convey "patrician" more believably than an American actress, especially now that the trend is for 50-somethings to try to pass themselves off as 30-somethings.
In any event, it's difficult to imagine anyone else in the role of Liz. Really, that's how I feel about all of the roles on DS, which makes me wonder if I'll be able to enjoy the new movie, no matter how well it turns out.