This is probably the most Occam's Razor-ey theory I've heard--Mexico was designed around the Eliminator, after all. Maybe FH6 was initially slated to be set in Germany but Martin Brown (who I've always felt to be more of an average games industry executive producer tbh) believed in the Eliminator's potential as a unique potential alternative to Fortnite and suggested moving it to a more diverse locale as part of a seemingly studio-wide effort to make the game more of a live service.
But I think the evidence lines up for FH6 being set in Germany more than these license plates would suggest, now that I think about it. Ever since livestreams ended with Horizon Track Day, a vast majority of the cars they've added have been from German, or at least German-owned, manufacturers:
Horizon Track Day: The four Porsche 992 911s, of course
Horizon Holidays Mix-up: BMW M5 CS, Bentley Flying Spur (Bentley has been owned by VAG through Audi since 1998, so despite being headquartered in England, it's essentially a German-owned manufacturer), Mercedes-Benz E55 AMG--sole exceptions to this being the Cadillac CTS-V Wagon and RZ34 Nismo
Horizon Extreme: DeBerti C10 Slayer, American-built so doesn't count
Out of ten cars added in the three months since livestreams ended, including Horizon Track Day, the Series they ended on, seven of them were either German or produced by manufacturers owned by German automotive groups. Horizon Extreme has obviously thrown a wrench into this, but I still suspect that the Slayer was left out of DeBerti's representation in FH5 because of a stubborn prior owner--though this suspicion largely depends on its model ID in the files, so it may be invalid. Midnight Muscle probably stands as the biggest question mark in all this--that Series title implies American cars--but considering that the Developer Direct falls on the same day this Series's Spring events start, which is roughly around when we've been getting newsletters, perhaps we could expect German GTs coming in to celebrate an FH6-set-in-Germany announcement?