I imagine Nintendo is hesitant to add wholly new characters from existing 3rd party series, not because there's an arbitrary rule saying they can't, but simply because if they have the resources to license and develop a new fighter, why not just make it from a new 3rd party series? Echoes like Ken are probably the most we'll be getting for awhile.
Not that the 'iconic' argument is all that substantive, but... come on. Between Sonic and Final Fantasy, it's not that hard. Say what you will about sales numbers, but the wider, casual side of the Smash audience will definitely know Tails or Knuckles before they know Chun Li or Sephiroth. Sonic isn't the most hip thing inside the hardcore Smash bubble, and Cloud is definitely a big deal, but you'd be hard pressed to claim any third party franchise has more recognizable characters than Sonic.
Of course, we got Solid god***n Snake of all characters before Sonic, anyway, to put into perspective how little this matters.