My current speculation thought process for whom FP11 is boils down to these factors:
1. If the character in question is from a franchise already repped in Smash with playable characters, do they have enough content to actively justify a spot? A lot of people have argued for Waluigi or said a Gen 8 Pokemon is inevitable, but the problems they both share is lack of enough content beyond a stage to justify inclusion. Waluigi doesn't have much for spirit board content or music tracks specifically related to him, which makes him more likely to be a bonus character akin to Piranha Plant as opposed to a fighter pass character. Same is true for a Gen 8 Pokemon, especially when you also factor in that the Sword and Shield spirit event that happened very late in the first fighter pass, while FE3H had nothing until Byleth's reveal and release.
2. If the last character is female, would her outfit need to be drastically altered to fit in? This is largely looking at potential characters like Shantae or Morrigan, with the former getting in Smash Ultimate as a spirit based on the Half Genie Hero artwork, but not a single alteration occurred. While some female characters like Camilla from FE and Mythra had their cleavage covered up, the large majority of the censorship that CERO often enacts is around the legs and waist of the female characters (recall how Sakurai mentioned that they kept bugging him regarding Palutena's skirt and how that nearly made them miss the release deadline for Smash 4). Unlike Mai Shiranui whose based around seduction and distraction with her body, Shantae and Morrigan really aren't and the latter could get in relatively unchanged.
3. Will the character be a major 1st party or 3rd party pick or a more lowkey one for those two categories? This is by far the most contentious part as patterns are constantly upended or shattered through the speculation cycles for Ultimate. My current leaning is towards a more lowkey 3rd party pick or a 1st party pick like Ayumi Tachibana, but I could easily be wrong about this.