As I stated the Targon lore has left me confused at this point, I myself am unsure whether I want the interstellar empire aspect of it to stay or go.
From the 'Avenger viewpoint', when you have 140+ champs and have only explored 1/6 of Runeterra, it makes sense that Targon's empire would go unexplored. The Vastaya and Darkin updates are proof that there is room for these other and newer races in Runeterra's history, without the need for other planets to be involved.
However, from the 'Thor viewpoint', these are champs exclusive to Targon, it makes no sense that you have this potentially enormous background behind these characters and never make use of it. Yes, "Asgard" and "Midgard" are the main focal points however the other realms can be used to produce an array of characters and plot available for story telling. Whereas from the 'Avenger' point of view Targon's interstellar empire is just a plot device to explain why entities like Sol and Bard aren't on Runeterra 24/7.
Soraka on the other hand introduced the idea of celestial beings that oversee Runeterra and gave a reasonable explanation for why a celestial being like Soraka would concern herself with the mortal races of Runeterra. A concept of celestials much more in line with Riot's statement to keep the stories Runeterrra focused (and what I was actually hoping we'd eventually get from the Targonians). So I am really happy how Soraka's lore turned out and her connection to Targon but I have no clue what to expect from Targonians themselves going forward, this is due to the lack of info we have on them.
This is why Targon Prime, the beings that captured Sol and the ones in charge of said interstellar empire, should be fleshed out, and I disagree that doing so would result in the region becoming uninteresting. Mystery should be as a result of ambiguity not solely as a result of a lack of information. Sorry for the long read, I'm really just hoping Riot knows what they're doing.