I've always seen the Shivans as an insectoid race, due to the appearance of their ships and the attack drones/shivens in the cutscene. So I've always considered them to have an insectoid mentality of working towards the greater good, and not being worried about their own lives. I know the explanation of why they attack civilisations is heatly debated, but I've always thought they simply attack races which are starting to become a possible threat to them. A sort of pre-emptive attack to stop them from developing the tech to be able to threaten the Shivans. That's why they're not bothered with taking land, as they aren't on a conquest, but more of pest control.
I'd like to see the next Shivan fleet focus on lots of Medium sized strong ships, rather than the traditional "boss" ship. Stuff which is supposed to swarm, overwhelm and pick apart bigger oppenents with numbers alone. Then again it's more likely we'd just see one huge arse Sath style beast, with the traditional claws which fire lazer b34ms.