I actually really hope we do see costumes, considering the circumstances. I think that's one of the reasons why I tend to feel appreciative towards the costumes as a whole; If they sincerely cannot get someone in as a fully-fledged character, the representation is appreciated. And now, after Sans got such a well-made costume, I actually wouldn't say no to other franchises receiving costumes that may not really be suitable for an actual character. (Example; I'm still totally jazzed about the concept of a group of FNAF costumes, since the characters themselves are ultimately suits/costumes for robotic exoskeletons. It'd be all-too fitting.)
At the end of the day, I definitely would
prefer actual fighters. But, understanding the amount of work and time/energy required to make something like that come to fruition, I enjoy the opportunity to play dress-up with the representation available.