Yes, these are some much needed things. Are there any immediate concerns with tackling this now? It doesn't seem like anything should be holding these additions up short of an implementation and proposal.
I think the best way to solve most of these problems is just to go with Joe's suggestion and use a method for these kinds of operations. A substring method built on some kind of particularly powerful Collection.slice method side steps a lot of the concerns I think most people have with a subscript based solution.