Note, Capital Metro is working on a similar proposal: https://capmetro.granicusideas.com/all/ideas/improve-functioning-of-this-site
Once an idea has been added, sometimes it goes to an in-progress state, sometimes it goes to an "acknowledged" state where it lingers forever. It's unclear in theory, but clear in practice that ideas that have been around for decently long will never be acted on. This leaves a bad taste to the site, as if you're throwing ideas over the wall and somebody on the other side is just checking off "yup, saw it." It can even retard efforts at change by providing a sort of "placebo change" process where people think that adding an idea to this site had some sort of effect, when in fact it had no effect whatsoever.
I propose instead, commit to respond to the 2 (or 1 or 3 or whatever your capacity is) highest-rated ideas at the end of each month, and at the end of 3 months, archive ideas as not popular enough. Other ideas can be responded to ad hoc if the appropriate people on staff see them. That way, 1) users have some understanding of what their idea needs in terms of popularity to be responded to, 2) users don't get an unrealistic idea that every idea will be responded to and therefore don't have to endure forever wondering whether their idea from 2 years ago will ever get noticed. It can also work to focus a community, instead of losing people among years' worth of old ideas.