This was mentioned briefly during the recent Reddit AMA.
I'd like to see the original Spintires maps return to MudRunner, either in a content update or just as certified mods.
I know the MudRunner maps are just altered versions of the originals, but they've been so extensively overhauled that they're often unrecognisable. Since they all have different names, I think the original maps could just be added back and then there'd be double the effective content to play with. Either as a future update, with achievements, or as a certified mod, where you could then make it clear to new players that these maps are from Spintires 2014.
The same goes for all the other bits of content that were in Spintires but not MudRunner - additional addons (some are which are already returning as certified mods I see) and customisable tyres on B/C-class trucks.