Well since no one seems to have an opinion on the matter I'll share mine.
1.) The old National Wonder system heavily favored compact play and punished wide play. Especially wide for the sake of soaking up land while not bothering to build up your infrastructure. The new system fixes that, the new system is generally compact vs wide neutral might actually favor wide a tiny bit but it doesn't really matter since you're usually well above the required national population when you get access to the wonder.
However 2 things however gets pretty dumped by the new system however and that is OCC (large part) and Venice (somewhat less). This would be easily fixable by making the capital count as double pop-value as long as you only have one city (If that is even possibly, just making a suggestion here)
2.) I feel like there is room for a few more NWs a food/growth-based one comes to mind also splitting east india company into one tradebased one and one goldbased (National bank?).
3.) I like the idea of national wonders in general
4.) Some of them are more fun than others, the percentual-based ones comes to mind.
5.)I would say generally no, it would be pretty easy to rank them in order from strongest to weakest.
National visitor center > Hermitage > National College > National Epic > Ironworks > Oxford (Could be placed higher since it is often used to bulb radio to get more early adopter tenents but after it is built it is just 2 writerslots) > Heroic Epic > National intelligence agency > Circus maximus (I'm counting bnw now because I don't remember what it looks like it the latest verson, it was completely op earlier on however) > East india company
While National visitor center clearly is the strongest NW in the game it is so late it the techtree that it hardly creates a balance issue.
Hermitage and National college however are creating issues, the national college is a classical era NW with the same effect of a modern era building (research lab). The Hermitage provides a 50% bonus to culture in a city, an effect a lot stronger than the strongest(and only) percentual based culture based building. That along with how the culture system works (you usually have most of your culture focused in one city) makes the Hermitage pretty damn insane, but that isn't all it does, it also got 3 slots for great works of art along with one of the easist themingbonuses.
The National Epic is pretty balanced/fun from my point of view, the effect is solid and consistant. Its effect is the same as the garden that is available one tech later in the tree, but I wouldn't say that matter much. Could maybe be strengthened by a flat bonus aswell.
The Ironworks I really don't like, it is powerful no doubt about that but I find entirely flat bonuses boring. I would like to see some of the flat bonus get replaced with a percentual bonus.
Oxford is another boring NW imo, the passive effects on it are so weak that saving it to bulb some powerful/expensive tech later on doesn't feel like a real sacrifice.
I don't have much experience with Heroic epic personally, in vanilla I rarely built enough baracks to actually get it. The effect on it seems solid however, but pretty boring, you could add something like a flat amount of GG-points when it is built to help people land their first general (maybe even let it give a full GG?)
National Intelligence agency in bnw suffered from having way too many and way too bad prereq buildings, however with those gone the wonder itself is pretty solid. It is hardly fun or powerful but it gets the job done. Also I have no idea what could be added to it.
Circus maximus in bnw was something you always built, mostly because you were forced to get the prereqbuildings anyways. The wonder itself is pretty boring but the CBP patch fixed that somewhat, but from what I remember it got nerfed again and is pretty weak again now.
East india company is imo the worst NW in the game, by a long margin. It gives you a flat 4 gold at the cost of never getting any traderoutes to your city ever again. If the wonder actually worked it would be fine but since I'm pretty sure the AI determine which traderoutes to send based on golddelta, not actually goldincome, meaning the EIC actually makes the AI less likely to send you traderoutes because you make more money off them. (this could ofcourse all be in my head but I can't remember the AI ever starting a trade with my city after I built EIC in it, rather sending trades to nearby undeveloped cities because the delta is better there).
Anyways even if this is all in my head EIC should probably be split into two, one(national bank) boosting flat gold and a goldpercentual increase and the East India Company providing tradebonuses (both ways) and probably a bonus traderoute or something like that.
6.) Kinda already gone over this but I'll guess I'll do it again
A goldbased one (national bank) giving flat gold and a percentual increase to gold income in the city.
A food or growth based wonder giving either a foodpercentage or a growthpercentage along with a flat foodbonus.