Considering the Queen's Indian big fianchetto
In my few first steps of the Queen's Indian defense, I always have used the classical fianchetto Bb7. GMs often play Ba6 with good success. I think I should adopt this plan, but what plan is it anyway? I should know before playing it. So let's have a try.
Obviously, this move gains a tempo because a pawn is hanging, delaying White's fianchetto Bg2. But its main purpose is the retention of the e2 pawn, x-raying both the pawn and the Rf1 after the castling of White.
By Qa4, White can force Black to go back to the normal fianchetto Bb7, but with other moves, Black will keep the Bishop there and block the light squares with c6 and d5.
This transposes the Queen's Indian to a Catalan position that gives Black a good and at least equal play. I think this is a good plan and I am going to play it next time when White will be avoiding to play Nc3 to which I have my Nimzo-Indian.