As the Dallas Burn, they played in the Cotton Bowl until about a decade ago, give or take.
Cotton Bowl is just too big for regular season soccer; with seating for 90,000+, a good game played before 25,000 would be lessened by the empty seats. Who knows if MLS will ever move up to such big stadiums. Of the stadiums used by Liga MX, only Mexico City's América play in a stadium close to the size of the Cotton Bowl, which holds 84,000. (https://en.wikipedia.org/wiki/Liga_MX
). Looks like Liga MX stadiums have capacity similar to typical American baseball stadiums.
The notion has been floated around before, even gaining a big write up by D Magazine (I think) that maybe soccer in Dallas would be better suited to membership in Liga MX....