2. The Real Mandarin Could Be Heavily Involved
One very interesting rumour about Captain America: Civil War suggests that the villainous antagonists of the movie could be HYDRA and the Ten Rings working together - the latter having the real Mandarin in tow. Iron Man 3 saw the Mandarin depicted as a sort of myth. Ben Kingsley's bumbling British actor Trevor Slattery portrayed the villain as a front and a means for Aldrich Killian to go about his nefarious endeavours undetected - and fans of the character generally absolutely hated the idea. Marvel Studios were all too aware of the fact that they messed up, so they produced a One-Shot short movie that saw Slattery being told, while he was in prison, that the real Mandarin wanted words with him - effectively retconning the Iron Man 3 depiction and confirming that the character fans wanted to see does in fact exist in the Marvel Cinematic Universe. It would be somewhat redundant to use him as the main villain in another Iron Man movie, but having him appear as a secondary villain in Captain America: Civil War would be a good way to satisfy the fans and this rumour is extremely tantalising as a result.