While everyone's been going crazy over the trailer to mega evolution act 4, you all seem to have missed the official summary

Alain's journey in pursuit of becoming the strongest once again takes him to the Kalos region. Wanting to become even stronger, he bets his Mega Ring and Mega Stone on being able to defeat 10 Mega Evolution users in a row. Alain throws him into battle. Mairin is worried sick about Alain. And Mairin's partner, Hari-san the Chespin, meets a mysterious being. It all leads up to something seriously major... We've finally reached the final act of the story! Don't miss the shocking climax!!

So it's straight up confirmed that this will be the final of the Mega Evolution Specials. If there aren't any new megas in this episode, there's a very real chance that there's no new megas at all in the next game. From the trailer you can see 4 of the 10 mega evolution users, being Venasaur, Banette, Alakazam and Houndoom. And if you remember the promotional image from Corocoro, it shows that Aggron and Pinsir are numbers 5 and 6. Fingers crossed that at least one of the remaining 4 is a new mega that will be revealed between now and then.

As for the blobby thing, not much is said about it. Just that it's a "mysterious creature" that leads to "something seriously major".