Having been away from home for a few days without access to my usual development computer, I cannot easily work on the code, but I have been able to work on the pakset. I have found a number of online sources of free 3-d models of aircraft whose licenses permit their use in at least non-commercial games (there are a variety of licenses: I link to the original file in the relevant .dat file). Some of the licences allow redistribution of the models themselves (which I am uploading to Github now, which should help people who want to produce additional aircraft from them), but some only permit redistribution of the rendered images, which will not be uploaded. I am somewhat hampered working on my Linux machine in that it is not compatible with Sketchup in which a lot of the free models are produced, and most of the automatic conversions to Collada or similar formats on the websites are broken (sometimes missing textures, sometimes missing parts, sometimes being entirely blank). I might be able to get further with that when I get back to my Windows computer at home, although I expect that I will need to prioritise the coding by then.
In any event, here is an overview of some of the new aircraft.

A Junkers F.13

A Boeing 274 and a Beech Model 18

A De Havilland Albatross

A Boeing 307 Stratoliner

A Boeing 737-100

A Boeing 737-300

A Boeing 787-8

A De Havilland Canada 3 "Otter"

An Embraerar ERJ-145

A Vickers Vanguard (cargo version depicted; passenger version also available)

A Vickers VC-10 (standard version depicted; super version also available)

Aero Commander 680 and Dornier 228

A comparison of the size of the Boeing 737-100, 737-900ER and 787-8.
There are a few others, not pictured, such as the Junkers W34, Lockheed Vega and De Havilland Dragon Rapide. With the exception of the Vanguard and VC-10, I have not added multiple liveries for these, as texturing is quite a substantial amount of work compared to converting them for use in Simutrans; however, for those who would like to create multiple liveries for these, I have uploaded as many of the models as permissible to the Blender Github repository.