That is a very hard question since we have only seen 2 games of them with the new roster and not too many before that.
U mean instead of niko or overall?
Ill just go with niko: according to former teammates and igls, niko is a very good teammate who can play a lot of the worse roles without complaining while maintaining decent numbers on paper, on heroic i think je was doing great at that, i bet it was a very hard decision to remove him from the roster.
OG is a roster that has basically only players (but niko and mb valde) who can still improve a lot, especially flamez. I have to say that i dont know all of nikos spots and roles exactly on OG,theyll also still change i bet but if u want to replace him you should go for an experienced support player who can play the "boring" positions on ct side. Also you need a guy that doesnt give much input in terms of secondary calling during a game i guess since 2 voices in valde and aleksi are enough as history has shown throughout a lot of teams.
So im thinking of es3tag who is currently not on an active roster, if he actually joins astralis to replace xyp9x which i hope it happens but at the same time i hope astralis stay together and see how everything works out on lan again, i could see the clutchminister being a great option for nikos spot. But i dont know if these 2 will perform much better since i cant really tell which roles niko will have in the end but both these players fit the calm and collected support player who just plays for the team at any given time and can do what u tell them to do as an igl