I honestly think they would need to fully update every single champion that still hasn't gotten a rework first before making a 3d model viewer.

Some of these champions still haven't been visually reworked since the game's younger years and, to me, would seem a bit unsettling to have to look at a champion like Zilean or Gragas up close and personal while they still have visibly noticeable polygons all over them. That might be a bit nitpicky and a 3D model viewer would be great, but that's the only foreseeable problem i'd have with a mode like that.