Timeline expansions are the bane of Paradox games. The wider the timeframe the more generic the mechanics have to be, and thus the less interesting. Victoria is the best in no small part due to the fact that it only covers 100 years and the devs could focus all their energy into squeezing as much quality and detail into that small timeframe as possible.
This is an interesting point. It makes me wonder if gameplay would be better served in the next generation by cutting it off even earlier, and making a completely separate game to come between it and Vicky. I'm not sure what the best cut-off point would be; that would depend on what the actual focuses of EU5 (let's say) and the new game were determined to be. Actually, this might not be a bad idea anyway; loads of people on the board say they never even play EU4 all the way to the end because they get so big and powerful it's just not interesting anymore. Maybe they'd prefer a different game to handle the later periods?
So while we're speaking dates, you might actually end the game at 1789 (sad though since your revolution mechanic is nice) and let Vic3 handle it from there. Historians do after all refer to the time of 1789-1914 as "the long 19th century" for very good reasons, so I think that should be the setting of Vic3.
While part of me salivates at the very idea of a specifically "Long Nineteenth Century"-focused game, I genuinely think that 1789 is a bad place to start Vic3. I don't want to have to sit through the French Revolution and the Napoleonic Wars every time I want to start a new game. Whether Vic3 (assuming it ever exists) begins in 1836, 1821, or even November 21, 1815, I really do think it should start
after the time of Napoleon.
Also, I can't tell if you were proposing that Vic3 should end in 1914 or not (implied by what you said, but not outright stated), but if so, then that leaves a huge gap between the end of Vic3 and the beginning of HoI. A gap that includes World War I, the Roaring Twenties, and the rise of Fascism and the Soviet Union and Communist China. Extending HoI to include all that would stretch its scope too far, and I don't know if people would even want a whole game just for that period. It's just as well to let Vicky handle it, I think.
(Of course, I have so little interest in a game that only focuses on WW2 that for me, the series effectively ends at Vicky anyway. I
really wish East vs. West hadn't been killed. I really wanted to see what a Paradox game about the Cold War would have been like, and if I were obliged to get HoI4 just to bridge the gap, then I would. As it is, I own no HoI games and have not decided whether to buy HoI4 yet. But that's me.)