As I’ve said elsewhere, this ‘collections’ suggestion is a sub-set of tag filtering, which is something that has been discussed before and which would be really useful in d*. There’s an earlier discussion here.
Being able to filter in various ways, for instance:
tagA AND tagB
tagA NOT tagB
tag from certain user(s)
tag excluding certain user(s)
etc.
You could have a dedicated stream for saved searches, and the stream for ‘tag from certain user’ could act as your ‘collection’ for that person, if I understand the G+ implementation of collections correctly.