From what we've been told, it isn't that Myung has no ideas, but that his ideas are more sections and parts than fully developed ideas.
That is my understanding as well. And it is typical in a band for there to be a couple of guys that are like that. And it is also typical that one of those guys is often the bassist.
Mangini, I'm at a loss for what his writing process is. They made a big deal out of jamming ideas of each other during the drummer auditions, so it would be hard to say he can't be there adding to the spark. Why, as Jordan put it, minimize the writing sources?
Not sure what his m.o. is for writing. But I get the sense that (1) he isn't and has never really been a proactive songwriter. At his level, I'm sure he
could write and/or compose music if he wanted to, but I just don't get the sense that he has ever really wanted to in any of the bands/projects he has been in. (2) He seems to be a pretty deferential guy. That came through in the audition videos, and I have seen that thread run through things he has said in interviews and social media posts since then. Of course, it could be that the editing in the audition videos made it seem that he is more deferential than he actually is and that I am now just reading that into other things he says. But that's my perception anyway.
As for what was revealed in the audition videos about the writing process and what they were looking for, I have a couple of thoughts: (1) They wanted someone who
can follow along with them jamming because that is how they like to write, but that does not necessarily mean that they want/expect the drummer to do more than create drum parts as they are doing that. Or maybe (2) That is what they were looking for at the time, but the writing process has changed/evolved significantly with Mangini in the band and Portnoy out, simply by virtue of the fact that they wrote most of the music for ADTOE without the new drummer just because that felt right at the time, and maybe that took them in a different direction that JP realized he liked. And then, two albums later, the inspiration struck for him to sequester himself and Jordan to write TA as a duo. So it is possible that maybe despite what they thought they were looking for, the writing process organically evolved in a different direction after they actually brought Mangini into the band. Of course, both of these theories are pure speculation on my part, but either seems like it could be perfectly legit.
In any case, when I see them in May, I am going to try to do my best to remember to discuss this with them and try to get more of a handle on the nuts and bolts of their writing process, and will share whatever I can about whatever I learn.