Guardians of the Galaxy didn't need any character to boost them.
Tony will be in Peter's movie mainly because he's the one who blackmailed him into the battle against team Cap, and tailored his new suit.
Guardians of the Galaxy didn't already have 3 poorly received movies behind them, with the last one having made the brand so toxic that they were the laughing stock of comic book fans (except for FFINO defenders, for whom that film is still their inspiration).
GOTG didn't go in with their main villain completely ruined and reduced to a pathetic electrical thorn in the FF's side in the eyes of the public. GOTG were a blank slate with no real public perceptions at all. It's not the same thing with GOTG and the FF.
That Coldplay song "Viva la Vida" could be easily used to describe what happened to Doom, from being the top Marvel villain and regal monarch of Latveria to a becoming a drunken anti-social blogger who looked like a homeless man wearing a trash bag:
I used to rule the world
Seas would rise when I gave the word
Now in the morning I sleep alone
Sweep the streets I used to own
I used to roll the dice
Feel the fear in my enemy's eyes
Listened as the crowd would sing
Now the old king is dead long live the king
One minute I held the key
Next the walls were closed on me
And I discovered that my castles stand
Upon pillars of salt and pillars of sand