>>>Could there be some sort of rabbinic board that connects the frum YU type and the Yeshivish Velt.
What for? Other than having members from both groups what other function would that rabbinic board have?
There isn’t any internal apparatus preventing a MO rabbi from becoming a member of Agudah if he wanted to. There isn’t any internal apparatug preventing a Yeshvish rav from joining the RCA either. There isn’t any internal apparatus preventing members of both groups from joining the hisachdus. The reason why they chose not to do so is why their won’t be such a rabbinic group either. The people involved have too much uncommon ground to be in a joint group without friction with other members ever coming up. Therefore better not to try to begin with.
There are venues that venues that representatives of Agudah, the hisacdus and the RCA all attend when there is clear common ground.