I wonder if the Dark Brotherhood assassination of Titus Mede II might actually spell the beginning of the end for the Dark Brotherhood.
Think about it: He has ALL the criteria that, in the real world, has historically been the stuff of martyrs.
He was a well-meaning ruler. Half of Skyrim, as well as Hammerfell, probably think of him as incompetent when it came to signing the White Gold Concordat. But I've heard no one suggest that he ever acted in bad faith in anything he did, even with that treaty.
He was relatively charismatic.
He didn't do anything to deserve death.
Even Ulfric Stormcloak was afraid to declare all-out war with the Empire, due to their resources. It was for this reason why Ulfric would not march on Solitude during the quests "Bound Until Death" or "Hail Sithis." I can only imagine that what's left of the Dark Brotherhood has even less resources at their disposal.
The DB was already limping on its last leg at the beginning of the questline, since there wasn't a Listener for over 30 years, so they had few ways of learning of new contracts and even fewer ways of recruiting new members. The Skyrim branch of the DB was reduced to only three members, when only about twenty Pennituc Occulotus soldiers attacked. Imagine the full might of the Imperial Legion & Penitus Occulotus combined, leaving behind in each city ONLY enough guards to keep order in the cities. The rest of the DB, across all of Tamriel, wouldn't stand a chance.
Once the initial shock of the Emperor's death wears off, my guess is that the next Emperor would go George W. Bush on us to avenge his father, and sends leagues of soldiers to the farthest reaches of Tamriel, ordering them to use enhanced interrogation techniques on anyone who shows even the slightest signs of DB affiliation, including any evidence of the Black Sacrament, shrines of Sithis, or even so much as possessing a copy of "A Kiss, Sweet Mother." Find the locations of the various sanctuaries, use rangers, skilled in the art of stealth, to spy on assassins entering the sanctuary and eavsdrop to learn the passphrases, and then ... Blitzkreig! About twenty DB assassins vs about 500 PO agents; the assassins wouldn't live long enough to realize what was happening!
They would eventually find the Night Mother's corpse, and burn what's left of that corpse, so that no new Black Sacraments can be heard and the DB can never rebuild itself.
Future games would simply have a replacement for the DB, kind of like how the College of Winterhold was a replacement for the Mages' Guild. Maybe the Morag Tong would branch out beyond Morrowind (though how they would be tolerated in other provinces is beyond me), or maybe an entirely new faction of assassins would rise up.
I wonder if that might even be an alternative, in TES 6, to the Dark Brotherhood questline. You can choose good or evil; join the Dark Brotherhood and assassinate the next Emperor, or join the Penitus Occulotus and destroy your particular branch of the DB! Skyrim vaguely hinted at this questline when you were offered the chance to the Destroy the Dark Brotherhood, but that was only a single quest. And besides, the PO didn't have the personal stake that they would have if the next game took place in the timeline where the DB was successful in killing the Emperor. I'd like to see an actual quest line, with promotions and rewards that mirror those you obtain the corresponding DB (e.g. about halfway through the questline, you'd be given a powerful white horse with the same stats as Shadowmere).
Thoughts?