The late-game economy isn't broken, I think. Yes, it goes into deep recessions.
Guess what.
So did the RL economy in 'lategame'.

Before, it was doing things like recessions so deep that it was causing world-wide de-industrialization, to before the state it was in at the start of the game.

Immigration is still being near-impossible to balance. The PDM team had managed to fix it only a few months before AHD, but now AHD is out they have to rebalance again, I think. Seeing Toronto already half-Bengali in 1850 is a little weird. 2050, yes, 1850, not so much.
I hope somebody seriously modifies Railroads to make them historically difficult. I mean, seeing both the Trans-Canada AND the Trans-Siberian by 1948 is a little weird, considering that the Trans-Canada was finished in 1885, and the Trans-Siberian in 1891, roundabouts...
And when the railroads were built in-game, they would be built everywhere regardless, not just in select few places, like historically. iirc, the US North had 10x as many miles of rail as the US South. This isn't represented in-game.

That's my personal pet-peeve, of course