1. I guarantee most people who play this game and buy this game don't give a damn if Sub Zero is a poster boy of MK. This game doesn't need him. At best, it's a compliment to Boon's product
How can you guarantee that most people don't care if he's in the game? Maybe people will still buy it, but if Sub-Zero, or another MK character makes the cut, this can effect the sales of the game overall. It worked for WB and NRS in the past games, so it would only make sense for them to do this again. And Sub-Zero can sell a lot to them. YOU might not care, but a lot of people do.
2. MK vs DC 2 sounds like a bad idea just based on how poor MK vs DC 1 was. It should be left alone.
That's not the point. Ed Boon and NRS don't wanna make a new MK Vs. DC because they wanna make different games each time, Ed Boon said that in an interview last year. it has nothing to do with MK Vs. DC, and putting MK characters in IJ1 or 2, or even putting DC characters in future MK games, doesn't have anything to do with it either.
3. I believe Scorps sells success is overrated. IGAU 1 DLC wasn't great. Scorpion came out when IGAU was hot (new game, CEO, EVO, summer vacation for most kids) Lobo, Zod, Zatanna, Batgirl, MMH isn't exactly a DLC to many people would buy all together. Not everyone is a fan of each. Even if Scorpion was the hottest sale, doesn't make it good for the future of the IGAU series. Now should we expect a MK character in every Injustice game? That's dumb. It's played out and keeps the content corny. Scorpion was a nice surprise but we when it happens every game, theres no thrill to that guest character. Most of IGAU1 guest appearances were surprises. A lot of IGAU2 guest will be mostly request I believe.
Overrated? I'm sorry but that's just stupid, Scorpion was still a hot seller and it sold the most in comparison to the other DLC chars. It has nothing to do with summer vacation or tournament schedule, Alien was the 2nd best seller for MKX alongside Predator, and when Alien came with the rest of KP2, there was no EVO, CEO or a vacation in that time. You just throwing things in the air. And yes, not everyone is a fan of those characters, but more a fans of Scorpion then any of the other characters in IJ1.
As for guest characters in the future, considering that they do sell a lot, then yes, they will keep coming in the future, whatever it's an MK character for IJ, a different character (for either MK or IJ) or even a DC char for MK (Joker and Darkseid can work in MK for example). And yes, it is good for the future as it makes them more money to make more games and more content, including the type of content that you want, which is more MK characters for MK games and DC characters for DC games.
and let people voice their opinion. The fans have a voice and they should use it. The game is already out, but people can still make request. Theres 6 more DLCs out and I'm sure people want to see their character in. Why be against that?
It's not being against voicing your opinion, it's that you need to accept opinions of other people besides you and people who think like you. I for one like that fact that we have MK characters in IJ2, and I think it would be interesting, like I said, to see DC characters like the Joker or Darkseid in MK11 and such. And if people do buy these guest characters and these make the most sells for each game (Freddy for MK9, Scorpion for IJ1 and both Predator and Alien for MKX) that means WB and NRS take the opinions of these people into consideration as well. Of course they can't make more guest characters then actual MK characters or DC characters for their respective games, as it's also about getting the license and the rights to use such characters (for chars from other franchises) and of course they have to stick to the universe of each game, but they can't just take them all out just because one group of fans of don't them, as the other group do want them. And if you care about letting people to voice their opinions, you need to allow these people to voice their opinions as well.