With “guarantees” I meant things like whether you want to have perfect forward secrecy, or whether you want to provide some degree of deniability, and so on, not so much what kinds of guarantees you’re relying on although they’re definitely also good to keep in mind.
“As secure as possible” is a very all-encompassing goal which doesn’t really say much – what I was trying to get at with my point about the guarantees you want to make is that you’ll want to have a clear idea of what you actually mean with “as secure as possible” so you’ll know what sort of eg. architectural decisions to make before you do a lot of work and paint yourself into a corner.
It’s a very ambitious project, but I can guarantee it’ll probably be very interesting to work on and you’ll learn a lot regardless of the outcome, and I’m definitely rooting for you.