Ok first off im pretty sure they didnt mean she would "become" wolverine as in be a guy...they simply meant she would take up his mantle as the wolverine. She would still be her character Laura but she would simply replace him,which is what they meant by the easiest one to play wolverine cuz her story with him has already been established

Besides the fact that wolverine was not the same wolverine from the original Xmen so THAT would still work in any storyline but in turn regardless of which Wolverine Hugh Jackman was technically wouldnt even matter because how do you know which Sabretooth was there. And as we saw 1000 different variances of deadpool..doesnt that mean then that there are tons of different variances of EVERYONE..hence, any storyline from any media form of said characters wouldnt matter or offend any backstory because any one you would come across in the void could be from any number of storylines that god knows what they were...right? Point being..they could put ANY character they wanted in the movie at any point regardless of what movie storylines we have seen thus far and it wouldnt matter so in that aspect as they put it in a review I previously read...they made it that they can bring in and out ANY character from ANY storyline they want to play with,let's say, in the future. So I can only imagine if they so choose to continue the deadpool franchise...they can quite literally do whatever they want and also have any actor they choose to play the role and can easily explain it that they are from wherever and whatever storyline and also besides that they could be from ANY time from said storyline. So to go back to the Sabretooth thing...even if he was the same sabretooth and the same wolverine from the one where they were friends...they could be from any TIME from that universe. So once again it wouldnt matter because what if they were from that universe but they could be from the past or future of that universe if im not mistaken. So anyway you want to look at it it plain and simple just doesnt matter.