I have some ideas on this topic.
My mindset is split into two parts:
1 mindset is:
I want to feel scared and sort of powerless because there is a more or less "huge" battleship unloading a huge alien attack force of unknown composition and power right into my precious installation.
That sense of fear would come from: "the general theme of the game", and the frightening thought of loosing all that invested time/money and experienced personnel, or at least having parts of the base UN-operable for some time after the attack (in the event of a successful defend) -with the costs associated.
2nd mindset:
The Xenonaut base is our "hometurf" and should feel like it.
I want to be able feel like I have the upper hand at least in some way.
Maybe in intelligence -cameras, sensors/alarms. Or maybe in some sort of action -being able to alter the battlefield to my advantage, deploy robots/drones/defense either static or active. Or perhaps some sort of element of surprise that would demoralize or at least put the aliens off their A-game, without seriously unbalancing the game. This, even after I, as the player have acquired all weapon tech in the game.
I think this difficulty balance is important in all aspects of the game, and the unquestionably hardest part is to find that balance.
Fun can come in many ways, but being teabagged by aliens in one of my bases after thinking I have done more then enough to be able to at least stand a chance is often an OFF-putting factor, at least for me. And definitely leads to a "load-and re-load game" sort of thing for me.