I have a suggestion regarding new compound queries, and to explain it, I will give you one example of many that I deal with regularly.
Recently I created a mailing list built upon a set of queries where I first had to single out ten different groups of accounts. I numbered each of the queries, 1 through 10.
Then I needed to add all ten queries together, so
query 11 = 1+2,
query 12 = 3+4,
query 13 = 5+6,
query 14 = 7+8,
query 15 = 9+10,
query 16 = query 11 (1+2) + query 12 (3+4),
query 17 = query 13 (5+6) + query 14 (7+8),
query 18 = query 15 (9+10) + query 16 (1+2+3+4),
query 19 = query 18 (1+2+3+4+9+10) + query 17 (5+6+7+8) = full list.
Now (Query 20) I can subtract accounts with various "do not mail" fields, creating my final list.
My suggestion is, on new compound queries, give users the capability of choosing exactly how many queries to Add together at one time (sort of like how many files to upload when updating a website).
In my example, I would still need to create queries 1 through 10, but if I could choose, in a single compound query, to start with query 1 and add 9 more, it could have saved me hours of work on this project alone (which is slowed down even more because I am constantly interrupted with phone calls, people popping in, and sporadic work on dozens of projects that I am have going).
It would have reduced the number of queries steps from 20 to 12.
Subtract might be another candidate for subtracting multiple queries from an initial query.
Intersect I have used only very occasionally, so I don't have much to say about multiple Intersects. I don't know how useful they would be, but if you're doing it for Add and Subtract, I suppose you might as well consider multiple Intersects so "all of the children are treated the same".