difficulty is MUCH more defined by any NewGRF or game script or AI you might configure.
Settings not included in the original also contribute way more to difficulty than just landscape, like map size(!), cargodist, town density, how easy town authorities are angered, plane speed, number of vehicles, breakdowns, service requirements, infrastructure costs...
In the light of these considerations, the difficulty presets were removed as they became no indication about the actual difficulty and as such were only annoying as they didn't do what they promised. The difficulty settings only made sense in their meaning with everything set to TTD values, no NewGRF, GS, anything modded AND map size 256^2.
It's very easy to say "I want difficulty settings". It's very much harder to actually come up with a sane preset of settings which resemble these - and define them such that most agree with your findings (also consider different play styles)
EDIT: another game uses string to allow easy exchange of settings: https://wiki.factorio.com/World_generator#Map_exchange_string - maybe such could be exchanged also via our online content (with the guarantee that they only work as advertized on the version specified). It could also include newgrf, ai and gs config.
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So I've quickly created an INI Merge project (VS.NET 2017) which can simply patch existing TTD INI with a new one (or just partial one). Nothing fancy, just a console, should be straightforward to use...