I do think the 70 dollars is a little scammy but it's a redone Campaign and Expansion plus a new multiplayer mode (which is supposed to be much larger than what factions was back in the day) that is where the money is going.
Factions was almost like playing a different game from sp. It was extremely tense for a console game that had elements of the campaign mixed with something like mad max.
If I had to guess I'd say that it will include elements that were basically simulated (like your infection levels and population health)into actual gameplay (maybe safe base camps or something like Ghost Recon and Division games have) as well as include elements of the infected in multiplayer. Like the sound of gunfire alerting dormant or roaming infected to your spot within the map. The infected outside of simulation were completely absent from the original Factions, which I thought was kinda crazy.
If the MP is really supposed to be a jump forward from what factions was, which was a good mp game that supposedly still has a following from 2014, then I could be okay with the price tag. If it is just the devs blowing smoke up the ass then naw bruh. The development of TLOUII which was deliberately misleading has made me question anything ND says or anything they are remotely tied to.
I think while the storyline seems to follow the beats of the original game (it doesn't appear to be the same kind of remake that the REs are or the FF7remake/sequel where things are wildly different) but just from having played the game so much it does look like there will be minor differences. Just having the controls and fluidity of TLOUII implemented in the original would be a big improvement.