From what i think... i reacon that Valve is too smart to try and include other peoples work into their own. To them it could be considered being Lazy.
More than likely we would be able to take a stroll through the Aperture Science facility, but i HIGHLY dought we would be able to use a Portal Gun on half-life 2; nor would we meet up with the character.
More than likely Valve would think of something even more intreging (spelling) and exciting than the Portal gun such as a WAY more improved version of the Gluon Gun from Half-life 1.
Portal would more than likely be based around the year 2000, which means that the character from Portal could have been taken hostage and stript of her weapon and was used to open up that portal in Episode 2.
Also... The Advisor, what i reacon what it does is that when it sucks the brains out of people, it can use the brains to find out in other words... what it sucks out... becomes apart of it's own brain
Everyone may have already of known that but yeah... i spoiled it just in case 
So Episode 3 will be 1 HELL of a fun game... lots of battles, more monsters, guns, vehicles, solves mysteries and we may FINALLY find out who the hell the Gman is.
I would LOVE to find out who that bastered is but... then again i want it to remain a mystery... hard to explain but... games that have mystery make a game fun.