Now THIS is an idea I have had for a while but lacked the skills (and time) to implement. Basically an IM client which does not log anything to disk by default (so there is nothing for anyone to recover about what was said or who was talking, great if you are in a country where the secret police like to seize the computers of suspected dissidents).
As difficult as possible to detect and block. Full end-to-end encryption with unique session keys (so even having the secret keys of all participants in the conversation AND a full log of the network traffic wont let you recover the data). High strength RSA for client to client authentication with strong protections against a MITM attack by a rogue actor (such as a police or intelligence organization)
And in my idea, it would be 100% open source and open spec and as widely distributed (both in terms of number of copies of the program and its code and in terms of geographical location of those copies) as possible. This ensures that its hosted in enough countries that if, say, the US government says "you can't distribute that, it doesn't have the backdoors to let the FBI listen in on conversations as required by CALEA", the program will continue to be available.