Just had some crazy ideas and one of the is borderline near madness but would work so well yet need a little more time to form but it involves Mew.
A rough idea I had is the MC is curious of what it is like inside a pickleball so one of the legends asks him to join her in their ball for some private time.
Another idea is for Eternatus that when she is first shown she has no breasts and is flat at first and never has had breasts before either (gender neutral?). Eternatus's form is effected by the type of energy she absorbs since she is also from space. Once the MC is getting her ready to be made love to (foreplay) a rumbling can be felt from her chest and when he puts in him extra large dick in her, BOOM BOOM, Mount Breastmores form in all their glory and Eternatus is surprised she actually has breasts for the first time ever. She tries touching them but they are really sensitive since they just formed and she has never felt this kind of pleasure before and does not know what to do. The MC reassures her and tells her to not fight those feels and go with the flow and help her with felling good. Once she gets on the ship being around so many different legends energies starts to change her body in all the right ways. Ex: being around Lugia has her ass get bigger plumper while just below Lugias ass in terms of quality. She loves how she is changing and wants to be perfect to the MC in assets.