A new type of unlock event that is said to be based off the original series. Based on the name this might refer to the Duelist Kingdom arc from the anime and manga. Additionally, we will be seeing a new type of duel and be able to unlock a new playable character, speculated to be Yugi Muto.
Let the Mayhem Begin [Mid-September]
Based on the manner of speech written on the text, the new event is most likely a variation of Rex Raptor, similar to how Super Joey was to Joey Wheeler and Elegant Mai was to Mai Valentine. Based on this, he will most likely be a roaming LD with drops different from the usual Rex Raptor.
New Upcoming Event [Late September]
A new type of event, that has never been released before, seems to be planned for late September.
The text reads a "mysterious tower", combined with the fact that this event will most likely precede a Halloween themed event, this event will likely be related to Soul-Absorbing Bone Tower.
"Face them with new strategies and techniques" might refer to a new type of dueling or challenges not yet seen in duel links.
An update planned early on in October, will be adding many new features to the world of Duel Links including:
Addition of a new world.
Addition of a new character
Addition of new missions.
Special Login bonus.
"Addition of a new world" might refer to the addition of YuGiOh GX, based on the recently datamined image.
Since "Addition of new characters" is planned just before October, this might refer to a Halloween themed character, Bonz maybe?
"Addiition of new missions" is quite vague, it may refer to a new type of mission that is yet to be seen. Or maybe new stage missions that will increase the stage cap. Worst case, it might only be referring to extensions on the lifetime missions(e.g. Login for a total of 1000 days).
"Special Login bonus" is very self explanatory. You login, you get free stuff. Although would this be permanent or a temporary event, similar to card flipper events.
Event: "Duel-a-thon" [Early October]
Duel-A-Thon will be held again in Early October. During the event period, you will earn points every duel, to fill your "Duel Meter". Gather points every day and get daily rewards, accumulate those points over the course of the event to get even bigger prizes.
A new character coming to Duel Links! [Mid-October]
Mid-October would be releasing a character "fit for this time of the year", if we are going with characters from the original series only, this almost 100% confirms Bonz as the new unlockable duelist. Adding to this, the dialogue written saying "Dueling me will give you a spook that will put you in your grave", further solidifies the speculation Bonz will be released.
They have themes to go off at least, even if they don't have specific cards. It's not like Mokuba used enough cards to build a deck, either. But they made decks influenced by his character. Dartz, Alister and Valon have all appeared as DLC characters in Legacy of the Duelist with pretty character-appropriate decks. Duke's pretty easy to build a deck around.