I would suggest this for EU5 - Remove the mana and have the following:
A tech tree that you unlock, and some have been unlocked by cultural traits (a hybrid of CK3 and Vicky3) - but also unlockable via diplomacy or purchasing rights (you buy a tech from X nation, but it can't be something say 2 techs away in the tree, it has to be next step, and you have to wait a minimum period before buying the next one).
Government expansion over time (kinda like Vicky 3, but done via estates - and also factions, finally combine the two!) So in this example, you have the faction of the guilds and the burghers estate forming a political movement, which vies for power together, against say the likes of the nobles and clergy who wish to join forces (to an extent). The more you conquer, the more than join the fold - such as local nobles of the region, so if you're Polish and you conquer through to the Steppes, you would have your Szlachta, but also you'd get tribes, cossacks, and if you went further afield to india, depending on where you conquer, you also get Marathas, Brahmins, Rajputs etc. So it's ever expanding and ever dynamic.
Development - this can be done via sprawl. Capital gets most developed, and select areas get developed also (but not to the level of the capital), everything further out reaching gets sprawl development. So the closer to your capital, the more development potential. If you take over a nations capital, you have the option to make it a sprawl city (perhaps, city of culture).
Religious Conversion - done via a combination of sprawl (like Centre of Reformation, starts at a fixed point, then expands outward) with a backup of clerics further helping the agenda. Replacing local temples with the faith equivalent (so, Church/Masjid/Gurdwara > Abbey / Mosque / Sahib > Cathedral / Grand Mosque / Grand Sahib for example)
Cultural Conversion - done via the CK3 method of a singular person on the province. The closer to your culture it is, the easier your can convert it.
Merge and Hybrid Cultures - done via CK3 method, but more natural. So You have the varying flavours of German (such as Saxon, Westphalian, Rhenish etc) eventually become German, or they hybridise with a culture that they border like French, and you revive Frankish or Lotharingian etc.
Admin - becomes a function of your court and advisors (hiring more than one, and recruiting specific ones would be a bonus), which becomes a bonus to your research of some technologies. This should include things to build like infrastructure - such as roads, canals, viaducts, bridges etc. Things that can aid in development, and troop transport and commerce.
Diplomacy - you have to hire as many diplomats as you wish, and their individual skill becomes what determines how relations are improved, also providing a chance to learn new technologies from far away lands. The better the diplomat, the more chance of success (they can also learn languages which would improve their abilities further)
Land Military - Generals develop tactics, aid in development of new military units and fighting abilities, and creation of Special units for each nation - such as Ottoman Jannisaries, Swedish Caroleans, British Redcoats, Spanish Tercios etc.
Naval Military - Admirals develop tactics, aid in development of new ships, flagships and such. Creation of special ships for each nation, such as Iberian Caravels, British and Germanic Crayers, Dutch Hoys or North Sea Balingers.
Golden Era - Only 1 nation in any continent can have a golden era at any time. Nations can have multiple golden era's, but if they lose a war, its an end to it, if their monarch dies, its an end to it.
Government representation - think like in IR...every region gets a governor that deals with things on a local level, whilst you deal with things on a national level, and you instruct them what to do.
More could be done for EU5, but that is just my take on things.
