Although all of the Witcher III's content has been free to date, the next expansion coming in October: Hearts of Stone, will add enough gameplay and extras that CD Projekt Red will be charging for it. It's going to be difficult too, which is why those wanting to try it out may need to get a bit of levelling in before it comes out.
The starting level for the content is 30, which some may not have even achieved on an entire playthrough of the game. To that end, while the game will be fully supporting of those that did grind their way to that level of play, for those that haven't, the new expansion will offer to fast-track them to level 30 to make it so they too can enjoy the new expansion without having to put time as well as money into it if they don't want to.
Of course if you're worried that you will have a much higher level character than that by the time the expansion releases in October, don't worry. Those who have battled through New Game+ mode will find the content infinitely harder for them, with scaled up difficulty based on their level. If you come in at level 60, it's going to be very tough indeed.
Hearts of Stone will release in just a few weeks time and will bring with it around 10 hours of extra in-game content. However this is just a prelude to the second big expansion set for release next year, which will add so much more.
What level did you guys reach in the Witcher III?