I would make a modern and open standard to replace email, fully encrypted and that would handle both voice and text and besides have a user interface that suits for short as well as long messages in a convenient way. =)
And if I get some time over I'd start making a replacement for the HTML standard that is more suitable for todays web apps where the inherited semantics no longer means anything.
That is related to one of mine: an email/messaging client that doesn't suck. Or rather, multiple clients based on libraries for ‘mailboxes’ of various kinds (including those in which messages appear asynchronously, like mail and messaging servers, RSS, usenet, scrapable web forums, and so on). I don't know how continuous AV links would fit in this framework, though.
Another five-year software project I'd have fun with would be LLVM front ends for various historically interesting programming languages, and likewise code generators for dead or niche processors.
If I also had money for expertise and prototyping, I'd like to make a keyswitch that feels like an IBM Selectric (or at very least an IBM beam spring switch) but fits ISO's thickness requirement so that it's salable.
And if I get some time over I'd start making a replacement for the HTML standard that is more suitable for todays web apps where the inherited semantics no longer means anything.