Forza Horizon 3 had some amazing DLC! The Blizzard Mountain DLC brought snowy mountains to the Australian setting and The Hot Wheels DLC installed bright orange tracks and loop-the-loops over the ocean front.
I would love something similar to Hot Wheels again for Horizon 4 again. As for something new the series does a great job with wide open road racing, but besides Edinburgh, city racing is very rare. With that being said I would like to see them add another large city area in DLC to have more street style racing or just expand the existing Edinburgh area.
I would also love them to expand on the non-racing portions of the games, More businesses, housing customization, player customization maybe add a campy story you can play along with..but that may have to wait for Horizon 5.