It hit me that, by the end of this Fighters Pass, Smash will have almost 90 characters, which is insane. (Mythra's number 80, there are two more after her, and there are seven echo fighters, which brings it to 89)
If they wanted to get it to 90 by including an echo fighter in one of the challenger packs as an added bonus, who would you want it to be?
Presumably, they'd have to make sense (it would be pretty random to include Octoling in a Sol Badguy/Guilty Gear challenger pack, for example), so I'd think that they'd either be characters from the same series (ex. Eggman with Metal Sonic as a Sonic echo fighter - Tails could work as an echo fighter or a unique fighter, but if Rex and Pyra is any indication, they seem to be sticklers when it comes to Mii costumes) or the echo fighter would be a clone of the fighter that's included with different stats/hitboxes (ex. Crash Bandicoot, with Coco Bandicoot as an echo fighter - she wouldn't really work well as an alternate costume).
I'd be fine with either of the examples that I listed (Eggman with Metal Sonic or Tails as a Sonic echo; Crash with Coco as an echo of Crash) - I'd probably prefer Tails over Metal Sonic, but I feel like Metal Sonic would be more likely to happen.