Reynolds and Thomas are who you would naturally think of when looking at this battle but I think Cooper will want in on the action. Yes he is normally a slot guy but he has learned all the WR positions and is more proven than either Reynolds or Thomas.
The other thing that will impact this battle is whether or not Everett or Hemmingway can emerge so that McVay decides to run more 2 TE sets. Hemmingway and Everett may show they are capable of splitting out wide which will lessen the need to a 3rd WR and improve blocking.
If Cooper or Austin show they can do the slot job you could also see Kupp be the other wide out.
It seems like we ought to look at all our talent and when you take that perspective you find a lot of possibilities to replace Watkins production from last year.