I'd happily pay $10/month for a 'pro' level or whatever they want to call it. I use Twitter constantly, but have always preferred third party apps - Tweetbot is by far the best.
If the pro level killed ads, increased data privacy somehow, and fully opened up every API to third party clients that would be more than worth it IMO. For example, I'm tired of Tweetbot kicking me to the mobile site to view moments.