You know, Half-Life 3 has a lot of things to deal with... Gordon and Alyx going to the Borealis, the Combine attacking again, what happened to Mossman and who the hell is the G-Man.
Portal 2 ended with GlaDOS letting Chell out of Aperture, so... what's next? I don't know about Chell, but the Portal series are more focused on Aperture Science and GlaDOS, so maybe the new playable character could be Rattman?
Portal 3 may end with Rattman defeating GlaDOS and discovering the truth behind the Borealis. Even G-Man could pay him a visit (something like what happened to Shepard, that wasn't a happy ending).
But in the end, what if the end of the series is a final chapter about a crossover between Half-Life and Portal? Maybe Gordon Freeman using GlaDOS AI to infect the Combine systems and take them out. Alyx, Chell, Rattman, G-Man, all of them should be there to say goodbye after more than ten years of this. Manly tears will be shed.