The series' Denser and Wackier shift change is due to NETRICSA being corrupted.As has been noted, the series has been getting goofier, though chronologically. NETRICSA was installed hastily to give Serious Sam an edge when fighting Mental's forces, both in the current age and in the past. But, be it an unexpected virus, untested technology, or just having a powerful computer implanted in the brain, NETRISCA is damaging Sam's brain, and causing strange visions and hallucinations. BFE has barely any strange visions, though a few start to pop up around TFE, notably the giant-headed Croteam members and the pyramid of crates. A few more oddities occur in TSE, notably the Croteam ship that takes out the space ship. By Serious Sam 2, NETRISCA is heavily corrupted and causing massive damage, including Sam now able to hear NETRISCA, the fairy-tale planets, the game show, and the craziness with searching out the amulets, only to hallucinate seeing a pile of them at the end.
There's going to be a remake of 2Before BFE was released, Croteam remade the first game of the series in an HD version. It would not be a stretch to predict the second game of the series would get a remake as well to coincide with Serious Sam 4. However, the major changes will be making it mesh more with the other games in the series, namely replacing all the really wacky enemies with the original mainstays and using the same visual style as BFE. While this would make the game slightly more Darker and Edgier, it will still be the kookiest game in the series.