Nikki Valens is amazing and masks of nyarlathotep could easily have had its structure as a way to wrap up all expansions but other people have pointed out, I don't think they'll stop if they think they can release another and get huge sales so sales will determine everything. I don't think there should be Limitless sideboards but I also don't think side boards are worthless or bad as some people think. The only sideboard I think we really do need, if need means anything when it comes the board game expansions, is an Arkham Countryside board that perhaps had four locations that together had Arkham proper including more specific encounters with the Asylum and miskatonic University, location for innsmouth, location for Dunwich, and location for Kingsport. That is the Nexus of the world in Fantasy Flight Games version of Lovecraft Mythos and it would be a real Delight to have Arkham country as a sideboard. If the game is over now, I don't think they'll immediately jump to a second edition or something to replace it. Not only is Lovecraft games already saturated out there but even specifically with Fantasy Flight there will saturated. If they do release a 2nd edition later on, I think they will hopefully be smart enough to do what they did with Mansions of Madness and find some way of incorporating first edition materials so that people who bought first edition stuff, don'ts feel like everything is Obsolete and a waste. I really like that they took that approach with Mansions of Madness second edition and it was great that they included in upgrade kit automatically in the 2nd edition components. I think it's years off before there's an Eldritch Horror 2nd edition but hopefully they're smart enough to do that otherwise they will get a crazy amount of backlash and also I think they recognize that making it so first edition materials can incorporate into 2nd edition is one of the best ways to get first edition owners 212 upgrade so to speak
But Eldritch Horror and itself was sort of meant to be a quasi replacement or sequel to the first Arkham Horror and with Arkham Horror the living card game out sharing its namesake directly rather than just Arkham Horror files as Mansions of Madness and Eldritch Horror have, I think they really feel like between these two games Arkham Horror original has been replaced and supplanted and I don't think they'll be in a rush to try and supplant them self again
The nature of masks of nyarlathotep sort of being analogous to the final expansion of Arkham Horror could again mean it's the final expansion but it doesn't have to mean that. I'm sure they would be happy to try and surprise people and go against expectations and justify this as sort of a punctuation or way to break up the cycle. It's a cop-out but I give it 50-50 whether this is the final expansion but I really wish and hope they make an Arkham Countryside board expansion.
With future expansions, it would not be too expensive or difficult to include the personal stories of any new characters added at this point. And by doing so, they may feel their encouraging people to go back and buy masks of nyarlathotep to get personal stories for the other characters and the same way there are now two expansions that have Mystic ruins Decks that each supplement each other.