By the way, I think their rate limiting is fine for most third party apps. There is a lot that can be done with intermediate caching and the Streaming API.
1) Ads via the in-line feeds [removing ads is a ToS violation, same way the real world ad networks now is basically a js/link configuration...can sell CPC this way]
2) Charge for API calls.
3) Charge for "premium membership" that unlocks the api at like $10/user/year
--- For in-house development and/or SaaS software that interfaces with Twitter. Even something like $.10 per 1,000 requests would add up pretty fast for large deployments.
Example? http://freshdesk.com/integrations/help-desk-ticketing-twitte...
These people could be milked by Twitter. Its a SaaS webapp so they can protect their API access.
3) Charge for "premium membership" that unlocks the api at like $10/user/year
--- app piracy isn't an issue if a user is paying to unlock the API for their account
Also, if app piracy is an issue because they can get ahold of your API keys, that just means you need to route it through your internal API and secure things properly. It isn't Twitters job to fix the fact you don't want to have any infrastructure.
As the app grows the usage grows and twitter makes more money.