Rival system: While much more interesting than the old system it does have a lot of quirks. It's currently way too easy to get PP from taking land (which lasts for ages) making the system rather easy to game. The rival pool is far too small meaning you'll have to start declaring on allies or simply run out of people to beat up. Finally the limit to a single rival at the start of the game breaks a lot of the gameplay balance.
There might need to be a new system in place for truly great powers that forces them to play world police or something.
I suspect (well, hope) it will be calibrated a little as time goes on. The rival pool is way, way too finicky, you can't even set some neighbors as a rival. As Morocco, I can only set Mali (an ally, for now, but an utter joke since they're sub-saharan) and Mamluks (probably the second most powerful nation in the world. Also an ally) as rivals.
Castile is no longer valid for some reason (even though they have ME as a rival), nor is Portugal (who is my friend in this MP game, though I had him as a rival for a little while). Nobody across the Mediterranean has shown up at any point.
There's probably some weird criteria here that hasn't been fully explored or posted on the wiki yet.
The mechanic as a whole doesn't seem to play too well for nations that aren't in the thick of things though.
Westernisation: This is just ridiculous. You have to be so far behind in tech and since it scales up by size you can't be a decent empire. The added RR is also crippling and makes the whole game whack a mole for decades. Needs to be rebalanced to have governments, ideas (plutocracy, innovation), alliance with western powers and tolerance have an impact on westernisation speeds.
Westernising as Ragusa (why the hell is it eastern anyway?) was one of the most painful things I've gone through in EU4, I can't even imagine an Asian or African attempt. I heard NA natives have to be around 26 levels behind to start westernisation.
The major change is no longer getting Western units after westernising and due to the harshness of westernising makes it very debatable whether it is remotely worth it.
The tech difference change does seem to really hurt Native Americans. I think it could be an unintended side effect of the Conquest of Paradise changes, which nerfed their tech group and forced them to finish their native ideas to remain 'competitive'. If westernization would factor this in, it'd be a little better, requiring them to be a net of 17 techs behind. That's around 6 in each group, which is easily possible to have by the time Europeans start really showing up in North America.
Not exactly sure what the change was intended to accomplish, besides stalling advancement of African and native nations.
It doesn't actually hurt anyone else that much, even for the Asians, they only need to be behind by an extra few techs.
France: The BBB is back with vengeance. She can now integrate all her provinces in barely a second. She can also make an alliance with Castile almost day 1 since she will only rival England (for some reason Castile and France are not historical enemies) and then they play the merry game of destory europe together. The combined forces of europe can't slow her down and she'll be chewing on the middle of the HRE by 1555.
I'm seeing this too in my MP game, Castile and France being buddies. I hope it breaks up sometime soon, because it makes for a ridiculous powerbase in Western Europe.
This is probably related to the weird rivalry system too, so maybe for some reason France can't set Castile as a rival, or vice versa.