McCullum stepped down as England Test coach last month.
England's white-ball head coach Brendon McCullum strongly believes Harry Brook's time will come to lead the Test side, despite Joe Root receiving the nod following Ben Stokes's retirement.
Brook was the designated vice-captain until the recent home Test series against New Zealand where McCullum was still England's red-ball head coach and Stokes was skipper.
However, when Stokes was suspended for the second Test following a team curfew breach, Root was handed the leadership role.
In the aftermath of that series and Stokes's decision to quit international cricket, Root was re-appointed as Test captain by the ECB, enabling Brook to focus on white-ball leadership and his individual growth.
"Everyone knows the affection that I have for Harry," McCullum told Sky Sports. "He's a wonderful player and he's doing a fine job as white-ball leader. I think his time will come in the Test job.
"I'm looking forward to working with him and hopefully growing and developing him into the leader that he can become. At some stage in the future, he will have a very important role in that capacity. But for now, Joe will do an amazing job," McCullum opined.
While McCullum retained his job as England white-ball head coach, the ECB named his former New Zealand captain Stephen Fleming as the new Test coach. Fleming will officially take charge starting with the South Africa tour in December 2026.
McCullum heaped praise on his successor for the England Test team, highlighting their close trust and shared goal for English cricket success.
"I had a great time in my four years but now it's going in a different direction, I think Flem will be excellent," McCullum said. "He's a little bit like me, but better – that's how I would sum him up. He'll do a great job, and the ECB have done incredibly well to get someone like him to commit to English cricket for the period that he has. It's an outstanding achievement."
"We talk pretty much every day, we're close mates. We've done a lot together and he's one of those guys who I trust completely. We both just want what's best for English cricket.
"We want to set this up so that English cricket can be successful across all three formats of the game. That is going to take some give and take at times, we both understand that, but we will work very closely together and give everyone the best opportunity to get the success that we all want to see," McCullum added.
Joe Root's second tenure will officially begin with the three-Test home series against Pakistan, starting on August 19 at Headingley. Since Fleming will not arrive until later in the summer, assistant coach Marcus Trescothick will take interim charge for the immediate upcoming matches.