Civ 6 largely treats science as a thing you generate through investment. You have a specific district (Campus) with specific buildings (Library) which generate Science innately (adjacency or base yields) and you can then increase those yields via Policy Cards (e.g. Rationalism) or City States (i.e. Science City States, which give additional yields based on buildings). This is the main way to generate Science.
Civ 6 does provide other mechanisms to get Science, including trade routes (international trade routes and also trade with allies), indirectly through other districts and buildings (e.g. Free Inquiry, which gives Science from CHs and Harbours, or the Dark Age cards which boosts Science based on Holy Sites), and through Specialists, but these are totally overshadowed by Campuses and Campus Buildings, particularly once you leverage Policy Cards and City States. The only exception is Science through Population via Pingala, which is very strong, but of course limited to one City.
This "Science through Investment" approach is different to, say, Culture. You really can't generate all that much Culture from the Culture District and Culture Buildings alone - to really get enough Culture as the game goes on, you need to get Great People and then Great Works into those buildings, and or build Wonders next to your Theatre Squares. The result is that, personally, I find generating Culture way more fun, because the main source of Culture is inherently more interesting (i.e. TD, TD Buildings, and the race for Great People and Wonders), but also the main mechanism doesn't overshadow other ways of generating Culture such as Pantheons, Trade etc.
I also think the whole "Science through Investment" is a bit anachronistic. To me, it seems that until very recently, scientific advancement was more a product of trade and mercantilism and increasing liberalism, plus a bit of religion thrown in (think of all those monks that advanced genetics). The whole investment in Scientific Research seems to me sort of a more recent thing, and somewhat tied to the creation of the Military-Industrial Complex. I'm not sure Ancient, Classical etc. societies were spending all that much time setting up the equivalent of research institutes and spending heaps of money and resources funding research into better cross-bows. Instead, it seems to me science was mostly left to free enterprise, with a bit of government largess and patronage thrown in from time to time (think
Queen Anne and Copyright and Patents or
John Harrison).
So,
what to do?
Well, I think basically the ship has sailed on "Science through Investment" mechanic. Civ has had Science buildings like since forever, and so that's just not changing. And frankly, Campus adjacencies are really fun - maybe the most fun adjacency after Industrial Zones. So, I don't think that should really change.
Instead, first, I think
pushing back Campuses to the Medieval Era as people have suggested is probably the best option. You'd still get to build Campuses, but there would be a bit of delay, which would allow other routes to Science to have a bit more value. It would also have the added benefit of making Districts less frontloaded in the Tech Tree.
If you did that, you'd need to make a few other changes. I don't think making a library a City Centre building would help anything - people would just spam libraries rather than campuses. Instead, I'd rather see some other Tier 1 District Buildings maybe provide a Science Yield, so you were incentivised to build other things. But if we did have a City Centre Library, perhaps that could have some more onerous build requirements a bit like the Water Wheel does. Maybe something like a minimum Population requirement. Or perhaps a Library could be an exclusive building for your Capital.
Second,
get rid of Science City States giving Science to Science Buildings. This more than anything else is what makes Science Buildings just so powerful. I'd rather see Science City States give Science to some other District's Buildings (e.g. Commercial Hub Buildings) or just give Science to the Capital and / or Government Plaza. This would weaken already OP Science Buildings, but would also help balance Tall v Wide play, because there would be less of an inherent Science advantage for Wide Empires.
Third,
take another look at Specialists. I'm generally okay with where Specialists are, but I do think they could use a bit more balancing. I think one option might be to have Science Buildings provided additional Citizen Slots. Each Specialist therefore wouldn't be any more powerful, but if you could slot more Science Specialists overall then it might make Specialists (and Large Pops) more useful in general. And or I'd like to see non-Campuses Specialists also generate Science, e.g. maybe Harbour and Commercial Hub Specialists could generate some Science, either inherently or if you slot certain Policy Cards (indeed, maybe Rationalism could be re-tooled as a buff to Specialists).
for every 2 mountains - although maybe that's no fun?)
Yeah, that is the answer I expect from one, who has his strength in making the proposals ... btw, I really like most of your proposals, because they are usually well thought out and/or nifty ... 
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