Wait, what does this have to do with third party guest characters?
And also if you mean no Assist Trophy will ever be a character, then I must say I think that is a bold assumption. If you mean that.
Sakurai calls Snake and Sonic "guest characters".
People associate guest characters with Soul Calibur, which features guest characters that don't return in future installments (for canon and system reasons, which people don't seem to grasp...)
People assume that Snake and Sonic will not return because they are "guests"
Sakurai also calls Assist Trophy characters "guest characters".
He calls Magnus a "guest character" in Kid Icarus Uprising.
Ergo, if guests really are not allowed to stay, then no Assist Trophy that was in Brawl will return in a future Smash Bros. in any way shape or form, and Magnus will not return to the Kid Icarus series if it gets a new game in the future.
EDIT: What I mean by "canon and system reasons" is this.
In SC canon, the guests return to their home dimesions at the end of the story, and are not canon to the story anyway.
As for systems, let's take a look at who the guests are:
SCII: Link (Gamecube), Heihachi (PS2), Spawn (Xbox)
Well lookie here, three VERSION EXCLUSIVE guests that would make no sense on the other systems.
Next, we jump to SCIII which is for PS2. Do you honestly expect Link or Spawn to return? Heihachi could've returned, but he was sent back to his own time.
Next, we get SCIV, which again, could've had Heihachi and Spawn (and given the systems, PS3 and 360, they could've worked on their respective systems like before), but they were again, sent back to their own dimensions and times. Instead, we get Darth Vader, Yoda, and Starkiller, who again, leave after the events of the game. (And aren't canon)
Then comes Broken Destiny and Legends. Kratos and Lloyd Irving for the respective titles. Again, not canon and leave at the end of their games.
Now, SCV has Ezio. I don't know too much about the game, but I assume he left for his own time at the end.