Only two ideas really.
One- A remant path for the sledgehammer that allows it to repair barricades for maybe 30 base health (i think thats half the sledge base damage?), but lowers the damage of the actual melee by a moderate amount. Otherwise the actual "upgrades" further along the path would be the same as the default sledge, with the only change being the ability to repair. Would not "explicitly" need a model change, but if one was to be recommended, simply a color pallet change to blue and yellow would work. This idea mainly stems from there not being much outside of the electrohammer for caders to use later on in the game and, while the perf. hammer is fine and dandy for the most part, but an alterative repair tool that had a higher burst heal compared to a sustained one would be a neat addition. Would lead to an easier time repairing during hordes as well, if one person has a construction hammer (the potential sledge remant) and the other with the carpenter's it could greatly slow down horde rushes.
The other idea is a trinket (one of the expensive ones) called something like "barbed nails" or something. The abridged version is if you have this trinket, nailed props give zombies 2 seconds of bleed (2 stacks? whatever its called) every time a zombie melees the cade. For no other reason than it would give an alternative and additional means of damaging zombies when you are a dedicated barricader.