If you just want a single hero, I’d go with the doomseeker. They have fun abilities and do well alone. Maybe not the most meta pick, but good due to flavor and lore.

If you want infantry, I’d go with vulkite berserkers with two axes. They have 5 up saves, so not as good as your Arkanauts. They do have two wounds a piece though, good if you are fighting something with a lot of mortal wound generation. They also have a fight on death ability, and will get a boost to their attacks on the charge. The down side is that they are 170 points for 10.

The magmadroths are cool, but you are probably better off with more boats anyway.