It's pretty hard to make a broken army in 30k so long as you stick to a theme (no spamming spartans+ fire raptors+ Sicarians

)
Beyond that, if you only have a single calth set, pick up another one and built at least 2 20-man tactical squads and use the other 20 models however you please. a 10-man termiantor squad + 2 contemptors is nice as well. Beyond basic stuff we would need to know what legion you wanted to play.