If Noxus conquered Ionia then they'd gain a big advantage in magical warfare.

So you know how magic in Runeterra is rare? Well based on Tristana's story, magic is a lot less rare in Ionia. It is in fact infused with the land (and the result is that it makes their fish great). Because of this, the population of Ionia is basically basking themselves in magical "radiation" meaning that they're more likely to have magically capable people born their.

But it's not just a more magical population, but also a higher amount of magical experts and teachers to notice this stuff and teach it. I'm talking about the temples where people like Karma studied. Schools where people studied ancient traditions that were magical in nature. The magical populations of other regions are probably cut drastically because there are fewer people to actually notice and train these abilities. Just look at Shurima.

Before Azir was made in a bird betrayed, it seemed that Shurima had access to texts or the knowledge to flourish magic. Whether Xerath could naturally cast magic and just learned more through them or these texts could teach magic to anyone I'm not sure. But the knowledge did exist.

Fast forward to present Shurima, and the most modern mage is Taliyah who is scared of her powers and has to go to another nation (and then ends up getting better lessons in Ionia) to be taught.

Magic is rare because of a small magically gifted population and a lack of teachers. This is for everywhere outside of Ionia because they have both.

So if Noxus actually conquered Ionia, then they'd get access to not only a bunch of magical land that could give birth to future noxian mages, but also a bunch of teachings that they could utilize for their own purposes. Funnily enough, Noxus is probably the second closest nation to being magically inclined (ignoring the shadow isles and the void for obvious reasons) because their capital is built from the fortress of an ancient conquerer who had a mastery of dark magics. So if Ionia was conquered, the Black Rose may have tried to combine the two forms of magic (and possibly corrupt the very land of Ionia in the process). Making Noxus even more dangerous because they just gained a whole new advantage over everyone else.

Just a thought.

Off topic: interesting to note that most mages in league seem to be self taught. Further indicating the lack of magical teachings in Runeterra.