I'm really hoping that it's an update to the existing engine and not AC2. Assetto Corsa was still in early access only a few years ago, and if Kunos start pumping out a new sequel every few years, a lot of people are going to lose faith in their products. They've only recently finished fixing the bugs (some would say that they still have some work to do) and they haven't actually given us much time to play with the finished product!
They have recently taken on new team members so maybe, with their help, they've managed to add the features that people seem to want (weather, night time etc) to the current engine. Personally, these additions won't make any difference to me as I used them once in Project Cars / PC2 just for novelty value, and then never again. I personally prefer to have serious races in the daylight with a dry track, and leave the rain and darkness just for messing around. If Kunos are creating a sequel (and rendering the current game obselete) simply to add these features, then I'm going to be very disappointed!
As josap11 said, if it IS a sequel announcement, I hope it is compatible with current mods. I personally don't feel like starting from scratch with a dozen tracks and a handful of cars again. I also wonder if modders would just scrap their current projects, if this is the case, and just wait for the sequel. I hope not.
If this worse case scenario does come to pass, I think I'll just keep playing AC until GTR3 comes out and then not waste any more of my time on Kunos' products.