The MAPI mechanic (Market Access Price Impact) is an interesting idea, since it "simulates" the various difficulties of integrating a province into the national market. Maybe it was intended to simulate transport costs as well. 
https://vic3.paradoxwikis.com/Market#Market_price
Unfortunately, its implementation is really half-baked. MAPI bonuses were simply thrown in with random technologies, and the devs called it a day. Zeppelins can lower it, but not ships or trains? Can you imagine how big a Zeppelin needs to be to carry even a single percent of what a steamship can?

Therefore, I decided to change it.
Modifiers:
- Changes the base MAPI value from 75% to 70%
- Traditionalism penalty lowered from -15% to -10%
- The market capital gets +5%
- The road maintenance decree adds +5%

Technologies:
- Stock Exchange bonus reduced from +10% to +5%
- Zeppelins provide +5% prestige instead
- Paved Roads now add +5%

Infrastructure:
- Ports provide +2% with anchorage, +3% with cargo ports, +4% with industrial ports and +5% with modern ports
- Railroads offer +1% with experimental trains, +3% with steam, +5% with electro and +7% with diesel

This means that a fully upgraded coastal province with all the techs would have a MAPI of 97 (instead of the vanilla cap of 95). An inland province would get only 92, though. Additionally, the capital, or any province where the "road maintenance" edict is active may actually hit 100 if they're coastal (don't worry, anything over 100 is ignored).

While this is an improvement over vanilla, I'm still not entirely satisfied with it. However, since there's no way to cap MAPI, this is the best possible solution, and I think that this should work well enough. My initial idea was to create a MAPI "corridor" of sorts, with laws and technologies defining the upper and lower caps. Then, infrastructure buildings and better production options would "fill" out that potential. With such an approach, you would be rewarded for technological progress, law reforms and infrastructure projects, instead of a tech simply sprinkling some magical MAPI dust all over your empire. Well, maybe they'll change this system in the next patch, and then I shall try again.

Obviously, this mod may be incompatible with anything which changes decrees, government systems, production methods and technologies. If there are any more incompatibilites, post them in the comments.

I'm the author of this mod which was first released on steam's workshop, this is merely a mirror.

Please provide feedback regarding bugs and balance issues.