My question is if the reward of adventure always will be RoS packs or will be the more recent ones? If I buy the adventure after the release of the new expansion I get new ones?
Always RoS packs. Really, though, if you buy the adventure, your reason should because you want to play the adventure. I think its pretty fun! The packs you get from it are more a "complementary gift with purchase" thing than a value proposition.
f you're exclusively interested in improving your collection, you'd be much better off saving your $20 for the next big sale--usually they have a few deals each year were you can buy 30 packs for $20, or you can save up for a pre-release bundle for the next expansion and get a free legendary.