In the longer run, give up the old deep tube lines. They could be gradually replaced by Elizabeth Line-style tracks (larger capacity, fewer stops, good air conditioning).
Doesn't Elizabeth run even deeper than the current ones, though? Certainly seemed so in the elevators of the Whitechapel station. And it's not like you could replace the current lines without building the new one first and I bet there isn't too much empty space between the surface and the existing lines.
