Yes, space monsters were part of the original planned features for SE IV. It's just that the basic game has proven many times more difficult to program than expected. Those original plans were very ambitious!
I hope they will implement space monsters at some time in the future.
It would be great to have a "ruins" world that is really a trap: You land and your colonists unleash a 'planet eater' like the Doomsday Machine in Star Trek! First thing is does is EAT the planet just colonized, of course. Then it goes around smashing planets into asteroids and digesting them. It should be baseship sized with unique techs that make it damn hard to destroy. 'Neutronium' armor should be immune to even shard cannons, for example.
There could also be a "guardian" type monster like the MOO Guardian of Orion to protect any 'artifact' ringworld or sphereworld so that whoever finds it can't colonize it immediately, for example. (Yeah, this needs the ability to have a ringworld or sphereworld as an 'artifact' in the game map...
)
Lots of other 'random event' monsters are possible. They could be helpful in the same way that these and the Antareans were in MOO II. Even if you were "safely isolated" from other players and happily building up your empire you had better have some armed ships to deal with the occasional random attack. This helps 'level the playing field' and reduce the advantage of someone accidently getting put in a nicely isolated position while everyone else has to fight for space early on.