Yeah, an "arc" that will be hyped up and happen in one episode, the same thing for Dark Raphael in season 3 and the supposed Dark April. Besides, do you even have information that makes the mutant prejudice story as City at War, arguably the greatest TMNT storyline of all time? If you don't, then theories can't work. We can assume as much as we want and hype clouds our judgement; it's frustrating. Then again, I've been coming off a rough 2015 year.
I didn't say he wouldn't return, but I do think if they revive him (which odds are is correct, putting aside cheating time), this will prove my point. Like Mutagen Man and Karai, any sort of suspense or development from Splinter dying is moot. I'm not invested because of the book they published based on the Triceraton arc of season 3 and partially Beyond the Known Universe; the book may have come out first, but the scripts were done beforehand. I'd recommend the book, solid read and has stronger detail personally.
Like I said back with The War for Dimension X and as I said here, Invasion offing Splinter through injury worked because of what he's been through beforehand. Annihilation and season 3 didn't barring Fourfold Trap, Return to New York and if you want to make a case, Noxious Avenger. Doesn't compare to the examples of season 2 I said yesterday where he did more in season 2. Besides, if the Turtles barely morn for their father outside Leo moments, then why should I care for a character the others never discuss? It's writing 101; if you're not going to care writing an important character, then why should I invest?
Either Splinter is brought back like nothing happened, they kill him again and do the same thing they're doing now in space with minimal to no references, or actually develop the team from the tragedy. There are no other options beyond those three.