Ok, so let's start with Cetrion.
First and foremost - lack of creativity in moveset creation. Wind = stolen from Fujin, Fire = stolne from many fighters, Water = stolen from Rain, rocks = stolen from Tremor (and plants/scorns = stolen from Poison Ivy). If in base game she is missing and we have Fujin + Tremor + Rain and they have those moves in their arsenal, we are ok. No one will notice her absence.
Second thing - "Injustice-ish" vibe. She simply does not fit that much in terms of fitting to the MK Universe. She is more like character imported from some Marvel/DC world and rewritten to fit to Mortal Kombat. Of course this is personal opinion, but actually whole post is, so I don't think I need to state that in each point.
3rd thing - she is Elder God, sister of Shinnok, daughter of Kronika. Really? Do we need ANOTHER family drama and unnecessary connections? Do we need an ELDER GOD to step in just like that to the roster and to fight with mortals (and got beaten by them just like regular random jobber?). Not as a big bad boss. Just a casual character. Is that how you treat one of the most powerful beings in the whole Universe? Just throwing them into the fight like casuals? Now, you can counter that with statement "but Raiden is here and he is fighting and he never been a boss" but when he was added the times were different and we just got used to him. This addition just makes Cetrion a bit underpowered.
Next thing - her fatalities. Dear Lord.... She is outgrowing the planet and then blasting a beam (!) from her mouth (!!) with the diameter of 30 centimeters (!!!). SERIOUSLY? How can anyone come up with that ridiculous idea and how wasn't that canceled in the stage of drawing sticky figures on the whiteboard? Her 2nd fatality is slightly (only slightly) better, but the "Wolverine claws" are anti-climatic and total absurd.
What's next? Her concept in general. But that you may pin under "Elder God fighting humans just like that". There are plenty of realms to be explored, plenty of races and species and yet they decided to downgrade the Elder God - just ridiculous.
And then we have Kollector. Cool looking guy with plenty of tools in his
pocket backpack. And yes, his race can be explored even more! He is forgettable only if you see the story, but let's agree on one thing. There were no DECENT story that NRS created since MK9. He is mysterious, sneaky and cunning. He is chasing his gold and treasures but at the same time collecting things from different realms and places. His arsenal can be extended in next games and there are plenty of ORIGINAL additions to be given to him - NOT the moves that are stolen from other characters. That one thing is making him waaaaay more interesting than Cetrion can ever be.
Of course I respect your opinion and high five for this discussion, but if you are asking me - they are just different levels of new characters and I bet there are different people who came up with ideas for those two. In my world, the kollector-guy definitely need a pay rise and even more creative space for next ideas. The cetrion-guy should consider changing his work profile to soap opera screenwriter.
On topic - Yes, Mileena is great. Can't wait to try her ^^ waiting for more videos and really can't wait for her KK