They should add cavalry and motorised infantry attachments as support units. Historically for example this was done by Germans when they attacked into Crimean peninsula, they had too few mobile units to form motorised divisions, but they did attach mobile units into foot infantry units. A lot of HOI3 mods that have done research on this have added infantry and motorised infantry as separate combined arms, so you would get bonus from adding motorised infantry into regular infantry (if you want to) division, making that historical composition worth it.
Not sure, but i think Hungary and Romania also had small amount of cavalry or motorised infantry into some infantry divisions.
In any case, infantry divisions with mobile components were historical reality. And of course we can trace it back to pre-WW2, during WW1 German guard infantry divisions might have had attachments of cavalry, and before WW1 cavalry was far more widespread.
There are even stories of Roman Legionaries mounting up and riding into scene of battle, then dismounting. Early form of Dragoons AKA similar idea as WW2 cavalry. But i don`t have evidence to back that up.