--Master of All-- wrote...
I don't think Bioware will want to invest those types of resources into content that only part of the player base will experience. In short, I'd expect a lot of cameos, but probably not too many returning party members from either game. 
On the other had, BW set the default game in ME2 in a way that eliminated all the encounters with Urd Wrex, Gianni, the lab Asari, Conrad V, Rachni Errand girl, and Shiala. Now, if what you're saying is true, why wouldn't they set the default game (which players new to the series would use), so that more players see that stuff? On the one hand, you can say, "oh a new player wouldn't get who these random people are", but really, that goes for Anderson, Liara, Ash/Kaidan too.
BW doesn't JUST think about creating content that can be seen by the most number of players.They also want to "reward" repeat players and create replay value. They may still go the cameo route of course, because it seems to me that sometimes they plan a lot of things and just run out of time to do everything.