Army suggestions

Hey everyone. I'm got my first models for my army now. I managed to get the leviathan space marines for a good price and I reckon its a good starting point. I was wondering which units you guys think would be best for me to get afterwards. It can be generally good units or more salamander oriented units, I'm happy for all suggested.

I currently have:

1x captain in terminator armor

1x libarian in terminator armor

1x apothecary biogist(that i proxy for the primaris apothecary as looking at its datasheet, the biologis just looks so random and i cant make sense of any of it)

1x leutenant with combi-weapon

10x infernus squad 5x terminators 5x sternguard vets 1x brutilius Dreadnought