Glad she decided to join the KSS... she definitely came across as a keeper during the 12th gen auditions.
I think the big issue with her is that adding her to MM right now could mess up the successful formula that seems to be working... I'm sure talent-wise she can come in and be awesome, but whatever 'formula' this current formation has come up with could be disrupted if other people (no matter how talented they are) join.
Also, another thing is that there simply can't be too many front members/'aces' in one group... can you imagine an alternate reality where Sayashi, Oda, Miyamoto are members and this girl is added in the 12th gen??? The songs would need to be 15 minutes long to feature them, let alone to throw crumbs at everyone else!
Just my opinion but unless there's early retirements in MM, we are more likely to see Ruru in some new group. Berryz and Cute are just about to reach the end of the line, so there's a high likelihood we might see her in a new group sooner rather than later. But again, just my opinion, Tsunku might troll us and add her next year in a concert or something.
Also, I hope they do something with her and Sayashi where they acknowledge they went to the same acting school, something to establish a senpai/kouhai connection and maybe a rivalry element to it.