Other than ruling out Touka we're not really any closer to figuring out who the final villain is. Personally I believe it's going to be either Elefseria or a new Dragon God. The elements of the 5 Dragon Gods are based on the elements that correspond to the days of the week in Japanese culture(Monday-moon, Tuesday-fire, Wednesday-water, Thursday-wood, Friday-gold). However, there are of course two additional days, Saturday and Sunday, which also have elements corresponding to them(earth and sun respectively). If you're going to take inspiration from the days of the week there's no reason to stop at 5, so I think we might see 2 additional Dragon Gods after these ones have been defeated. I wouldn't necessarily buy into this idea if Mashima had just picked 5 random days for the Dragon Gods' elements, but the fact that he specifically picked the 5 weekdays while leaving out the weekend doesn't seem like a coincidence to me. It makes sense that the weekend dragons would come after the weekday dragons.
As for how this is going to go down, it's probably connected to the circular mural/tablet that Elefseria has which loses some of its color every time a Dragon God loses their powers. It will probably activate fully once all 5 have been dealt with, but the details are hard to guess. The most obvious option would be that the 2 additional Dragon Gods were sealed away and Elefseria is trying to free them because he wants to take revenge, since he mentioned that he learned dragon slaying magic to defeat a specific dragon. It's also possible that Elefseria could have been lying and that he actually serves the 2 Dragon Gods or he could even be one himself(probably unlikely since Natsu identified him as a human based on smell). Whatever the case I think he is going to play an important role in the final stretch of the series.
As for the role of the dragon eaters in all of this, I think they exist to give the rest of the cast something to do. The dragons are for the dragon slayers to defeat, but there are also other characters who deserve some focus. The dragon eaters can be their final opponents. Suzaku for example seems like a good swordsman enemy for Erza, and there's even a theme to it because Erza is Scarlet and Suzaku is named after the Vermilion Bird in Chinese mythology. Scarlet and Vermilion are different shades of red, so it's red vs red. In Addition to the Vermilion Bird there are also the Azure Dragon(Seiryu), the White Tiger(Byakko) and the Black Tortoise(Genbu), who would logically make up the remaining members of the Black Dragon Slayer Knights, with the Yellow Dragon(Qilin) being a possible extra member if Mashima feels like going there. Them + Reizen + Skullion's team can provide big fights to all the other good guys while Natsu focuses on the Dragon Gods.
I will say that the guild vs guild dynamic would make the dragon eaters the most obvious and practical choice for the final villains, so if Mashima feels like ending the series a little earlier he could just go for that. In that scenario you could also make Reizen an actual dragon, maybe Viernes like people theorize, and then involve Elefseria in the conflict by making Reizen the dragon he wanted to defeat. However, I much prefer the idea that there's something beyond the 5 Dragon Gods and that Elefseria is somehow involved in that.