I would pick Harvey ahead of Maddison, Henderson and even Gallagher. He works his socks off, positionally brilliant, brilliant technique and only 20. A forward thinking manager would see how important it would be to integrate someone like him early into the team. He plays like a mature head already and protects the ball brilliantly, can pick a pass and has a good left foot strike anywhere on the pitch. Fitness wise, he is exceptional given how many minutes he's played with no breaks and not looked off it at all. He has shown that he can make a difference off the bench and catch up very quickly to a games tempo. Southgate is playing far too safe when he has so much talent available to him.
Maddison was on his way to being one of the best players in the league before his injury, and has had numerous seasons as one of the most creative players in the league... He has 7 assists in 17 games this season for example, he is a top player and a bit more advanced than Elliott due to his age.
Elliot over Gallagher and Henderson most certainly, but the issue is, his competition for the role he best suits are some of the best players in the English team and league.
Southgate is not playing it safe in this case, Foden Maddison Bellingham Palmer are enough number 10's for one squad, the squad needs a Jones far more than it does a Elliott at this current point in time.