"Tweet nearby" & "Encrypted tweet nearby" & "P2P Tweet": These three functions would blow the lid off active user growth. Across the globe Twitter remains a key means for promoting events, demonstrations and encouraging political action. Give users the means to reach by locale (Concert), and securely by locale (Flash mob), and P2P (Iran, Syria, China) -- and sit back and watch the audience skyrocket.
Perhaps I'm misunderstanding your point on Iran, Syria, China, but you are talking about subverting authoritarian governments, right? Why wouldn't those same governments require Twitter to cut those off? I don't see how a centralized service like Twitter can ever provide that.
Continuing to use US private companies as soft weapons will increasingly harm the reputations and global standing of these corporations. The military-industrial complex gig is up. Other nations know what's happening. It's merely a matter of denying the company access to a population while making it look not so bad from the PR side.