Frankly, collections are a great feature. However, as per my experience, it gets cluttered easily. An folders for storing collections would be great for this. Heck, even an 1-level folder support would be great.
Although, this would just make Collections a Better Version of Bookmarks. But, hey, Collections are great!
Collection Search Bar
Adding a search bar instead could also be considered to be an alternative way of organizing collections without actually making it an alternative for bookmarks.
Collections is really a great idea so it really strikes me as odd how there is no real way to organize them. I really hope some feature is added to do it.