Smithy
Like many other characters from Super Mario RPG: Legend of the Seven Stars, Smithy has been long forgotten. Smithy was the big boss in that title and he even put Bowser in his place when he took over Bowser’s Keep. There might still be some beef there, though.
Their past is the only reason why this might not happen since Bowser seems like the kind of guy who holds a grudge. But if they put their past behind them then they would be a dangerous duo. As long as he can keep his temper in check, of course.
But even once he loses it, Smithy would be a great ally for Bowser. All his different heads would force their enemies to think on their toes. Bowser has picked up a handful of new tricks over the years, but nothing like what Smithy will bring to the table.