We desperately need anvils! There isn't any good way to make a decent looking one currently, the round bit really throws things off.

Some form of trip hammer would also be good (maybe just the striker head? the mechanism is simple enough).

Bellows are something we may have to just imagine for a while, for now my blast furnace has the wooden and mechanical component but no fabric sleeve.

An open-topped barrel for water-quenching would be cool.

Finally, this isn't necessarily a smith item but cauldrons would be really nice!

Free internets to any intrepid modders willing to tackle these very important objects!