The limitation of not being able to recruit anyone from RunAway makes this much harder. If it wasn't for that, I would've probably chosen the core members from RunAway and filled the team with remaining stars in KR scene.
Since this is to make a realistic team taking into account of team synergy, I would probably pick key members from Miraculous Youngster and LGD as starters.
Explaining the starters first. I think the main weakness of MY is the main tank, so replacing Jiquiren with guxue would strengthen the team quite a bit. Also, I think Shy is the best flex support in Chinese scene (zhufanjun is almost as good, but I think Shy is slightly better at Zenyatta and Sombra).
As explained above, I chose the starters not based on pure individual talents, but chose them considering the team synergy. So clearly, there are better individual talents available. So I think picking this top individuals filling in some of the weaknesses of the starters would be a great way to wrap up the roster.
aKm is one of the best, if not the best, hit-scan available. As long as he can play well with the team, he would be a great addition to any team. Also, he has shown his skills on Pharah as well, which neither of the two DPS in the starting line-up clearly displayed.
Another weakness of the starter is that although leave plays Genji from time to time, his Genji is not comparable to the top Genjis in OWL. So including ArHaN would complement this team quite well.
FCTFCTN is probably the best option left for main tank. KnoxXx is also a viable option, but I think FCTFCTN's Rein is better.
I think Void is the best D.Va outside the OWL. Void's D.Va and LateYoung's Zarya is probably as good as, if not better than, top flex tanks in OWL.
Fuze is amazing on Lucio and is also decent on Mercy. Wakawaka is another option, but I think I would choose Fuze over him because I think Fuze would have better synergy with Undine as they have played together.
Undine is very good on Zenyatta and Ana, and he is one of the best on Mercy. Greyy and Roolf are also viable options, but I would choose Undine over them due to better synergy with Fuze.