The 1989 season had the most entries teams and driver wise in the history of our beloved sport and had beautiful cars and liveries on pretty much every car of that season.
To start this project i had a look at which kits do exist in 1/20th scale:
Mclaren mp4/5 - Fujimi
tyrrell 018 - Studio27
Williams fw12c - Studio27
brabham bt58 - Studio27
lotus 101 - Modeler's
March cg891 - Studio27
Benetton b189 - Studio27
Minardi M189 - AMC
Ferrari f189 - Tamiya
Zakspeed zk891 - Studio27
Onyx ore-1 - Studio27
(i know some exist from other brands like the Tyrrell (Factory tz) etc)
so far i have collected 9 of these, missing the Tyrrell (can't decide on which version as i like them all) and the Minardi (just very hard to find) the cars not existing are:
Arrows a11
Osella fa1m
BMS dallara f189
Ligier js33
Larousse lc89
Coloni C3
Eurobrun er189
Rial arc2
AGS jh24
First (not sure i should include them as they didn't appear at any race)
ideas for making them would be conversions, 3Dprinting or hoping a manufacturer sees this and produces one of the missing ones ( we can only hope they do

as far as the conversions go these should be doable:
Arrows a11 - Tamiya williaws fw13b/ benetton b188 ( williams for the main part engine and frontwing are perfect from the benetton) the only problem would be decals. i do have a tameo 1/43 one so maybe a scan could do the trick. i also tried the route to 3d model the car but my skills aren't that well when it comes to surfacing/ smooth curves) but here is how the project halted: Ligier js33 - Tamiya Mclaren mp4/5b and using the studio27 decals for the js33b (livery was moreless the same)
Larousse lc89 - Studio27 Larousse lc90 (shape is moreless similar but would need other decals (maybe a scan of tameo ones?)
Rial arc2 - Fujimi Ferarri f87/88 the 1988 rial being called the blue Ferrari kinda made me think about doing a 1989 one. As far as i could see there are still quite some similarities between both cars. Decals would again be the problem, maybe even more this time as there isn't a 1/43 version of this one (as far as i know)
for the other cars i've yet to do research.
So far i finished the Ferrari a few years ago. The ones i'm currently building are the Zakspeed, Lotus, Benetton and Mclaren hope this sparks some more interest into this season as it would be great to share ideas about how to build them all