Honestly it is not surprising that we get so many ionian champs.
I mean ionia is largely based around asian culture,mostly japanese culture and lets be honest here,japanese culture is one of the more popular cultures,especially when it comes to fantasy settings.So pretty much any champ that is based on something japanese,chinese or maybe even on something indian,is almost automatically going to be an ionian champ(the exception is Xin Zhao,who is likely from ionia,but lives now in demacia).
Sure riot could do a jaguar warrior based on aztec culture,but almost no one would recognize such a character as a jaguar warrior and instead see them as just a random dude wearing the fur of a jaguar.
However pretty much everyone will recognize a ninja or samurai.
Meanwhile european culture is pretty much split up among the different factions,especially with the typical "medieval knights,kings,etc."-stuff being split up between demacia and noxus.
And in addition to that Ionia also is the faction based on mysticism and magic,so its not surprising that a lot of champs based on these aspects will also automatically go to ionia.
And in addition to THAT,we also get the fact that the ionian history of it nearly being conquered by Noxus also leads to a lot of different story elements,we get champs based on the idea that ionia is starting to pick up hextech tools,some based on revolution and most champion ideas for characters that were scarred by the war will also go to ionia.
That being said I agree that they should focus on other factions a bit as well,last year 3 out of 5 new champs were ionian.
Just because it is easy to make champs for a faction doesn't mean that they should pump out too many champs one after the other.
Especially since we are getting 2 ionian VGUs this year anyway.