A few thoughts about some of the 'new' mechanics in EU IV. For my purposes, this means anything introduced since mid-2017.

-- Army Professionalism -- In some ways, this is what I always wanted Army Tradition to be. A reflection of how well-drilled your troops are. Drilling is actually a mechanic now, and when a stack is set to drill (requires a general) it slowly builds professionalism, which grants bonuses to things like shock and fire damage, siege ability and merc cost. It works well enough, though the bonuses are modest, and a drilling stack is at lowest morale, so will need to recover for few a months after coming out of it, making it dangerous when suddenly attacked. Professionalism can also be granted through events, and as it grows, additional bonuses are unlocked, such as the ability build supply depots in a friendly-controlled province, meaning your troops will reinforce at the same rate as if they were on home soil.

After playing around with a bit, I started to ignore drilling, but since I can still get it through events it builds all the same, although more slowly. Requires Cradle of Civilization DLC

-- Absolutism -- Available starting with the age of absolutism, this is a reflection of how ruthless and efficient the government is, giving scaling benefits to administrative efficiency and army discipline. This is a very powerful thing, with the administrative efficiency lowering the coring costs of expansion. Combined with other late-game increases it means near the end you can take and core vast swaths of land. At 100% absolutism, your nation gets+40% admin efficiency and +5% discipline. There is however, a cap. How high it is, is dependent on a number of factors, with government type, government rank, legitimacy, religious unity, great power status and more all factoring in, as is the Paradox way. In the France run shown above, my max is 83 at that moment, which is still very powerful. Definitely something to pay attention to for all expansionist nations.

There is a 'strengthen government' button on which you can spend MIL points to bump your absolutism by 2 points.

-- Ages and Golden Eras -- The campaign is now divided in to four Ages (Discovery, Reformation, Absolutism, Revolution). During each age, there are a number of Age Objectives you can fulfill. When you hit three of them, you can opt to trigger a Golden Era, granting +10% Army Morale, +10% Naval Morale, -10% to all Monarch Power costs, +10% Goods Produced Modifier, and +5 Max Absolutism for FIFTY YEARS! That's fantastic. The catch is a nation can only have one Golden Era per campaign. So deciding when you enact it is important, aside from actually qualifying for it. Do you do it early to give your nation a headstart over others, but at a time when your modest nation can take less advantage? Do you wait until you want to punch above your weight, taking on a more powerful rival? The +5 bonus to absolutism only works in that age, so makes most sense to enact it then, but there are many factors in play here, and choosing the right time to start your Golden Era is important.

The ages themselves are fun and interesting, with each Age Objective you reach granting certain bonuses, which then end when the next age begins. For example, in the Age of Reformation, the following can be gained, but some are nation-specific and not all will be reached in some cases.

Religion Enforced Edict: Unlocks a state Edict that increases resistance to being converted by a Center of Reformation by 90% in that state
Coastal Barrage: Blockading the coast near a coastal fort increases the impact on siege progress more than it normally would
Religious Wars: All wargoals have -25% Warscore cost if they are being demanded of a nation with a different religion
Mercenary Discipline: All mercenaries you hire gain +5% Discipline
Powerful Tradeships: +20% Trade Power generated from Light Ships
Institutional Spread: +50% Institution Spread in provinces that have your state religion
Prestigious Conversions: Converting a province grants you +0.3 Prestige per point of Development in the province
(Spain Only) Spanish Tercios: -30% Shock Damage received
(Mughals Only): Mughal Artillery: -50% Artillery Cost
(Poland or Commonwealth Only) Polish Crown: +33% Goods Produced
(Persia Only) Persian Reinforcement: -30% Reinforce Cost

I really like the Age mechanic. It adds a lot to EU IV, and helps fill in the inter-war years with goals . Requires Mandate of Heaven DLC

I have to hand it to the strategygamer site, not only did they have that great rundown of DLC I linked earlier, they have this piece detailing the Ages mechanic. Good stuff gents!

https://www.strategygamer.com/articles/europa-universalis-iv-a-guide-to-ages/