Now that there are Regional Champions I don’t feel a 4th World Title is necessary.
What would be cool however is a title that links in with one of the current titles, an example being the WBC Silver Belt in real life which should give the holder a mandatory shot at the WBC World Title.
One of my biggest peeves in the game is having an injury prone fighter that wins but always loses his ranking due to being injured for so long. If he could win a Silver Belt and become a mandatory challenger as soon as he is fit it would give a whole new dynamic to the title picture.
It would also feel like you are “earning” the title shot rather than just hitting the top 5 and having 1/2 easy fights until the champion is free to face you (that is if they will face you at all).