Sure, I'll first point out that all of this is just how I would imagine it before this gets picked at. Because this is fictional I am filling the blanks for how I would see it.
If there was magically a Disney Texas I think it would end up being a less expensive option that would potentially open up the mid-western market for the parks. A lot of folks get priced out of DLR and WDW not because of the price of being there, but the price of getting there. With this in mind, though I don't think it'll literally be worse, I think it would immediately feel less magical (to me) than DLR and WDW. In regards to how it would change the current situation at WDW - I remove the context of COVID and them being "at capacity". It'd take long enough to build that regardless this (hopefully) won't be an issue. On any given day at WDW (with all four parks as an aggregate) they range from 35% to 65% capacity, which means that is definitely busy. My logic here is that though yes a new park would bring
more people in, the number would not be large enough that your overall capacity % would do anything to go down.
There are plenty of estimations out there about what the real capacity is for these parks in general, but the numbers that I've seen tossed out there are MK at ~100,000 guests, Epcot ~95,000 guests, HS ~75,000 guests, and AK at ~60,000 guests. If we pick a random percentage for the sake of math's sake lets say 60% (which was reported somewhere for 2019 summer %), that would mean there are ~195,000 guests in the total of 330,000 possible guests. If this 5th gate, let's just call it Villains, holds 80,000 guests I strongly believe that you would not get an additional 48,000 people to come to the park in a day to maintain that 60% overall attendance level. You see the conversation had here all the time, but there was a survey done by Insider where 46% of responders said that they either definitely would not (20%) or probably would not (26%) go to the parks due to the expense. Just making an assumption here, but I think this would either get
worse with a 5th gate or
not change.
Looking at all that info there, I then start diving into my own assumptions as to what this would mean for the time after the initial rush of the new parking opening as this would be an outlier to overall impact. I do not imagine more than 20,000 people (25% of the 5th gate/the imaginary capacity I gave it, I also think this is REALLY generous) would come because of a 5th gate. Honestly, I don't think more that 5,000 - 10,000 would pull the trigger because of this change after the initial hype... but I won't under sell it that much. With this math the overall capacity would go from 60% to 52%, and that 60% is on the higher end of the speculated range. I also think that dip would REALLY be felt in the parks, but that could also be just me.
I honestly don't think another park park itself is bad, but I don't think it really moves the needle as much.
Sorry for the long post lol, but you asked for it.