Deutsche Bahn plans Frankfurt21, a new tunnel and station below the existing main station for the ICEs. The high speed trains would save 8 minutes like that and it would help save time for the Deutschlandtakt. They don't call it Frankfurt21 (yet) and will keep the existing station, the estimated cost is 3.6 billion €, so make it 7 billions. An interesting fact is that the tunnel would have to go around the skyscrapers, because their foundations reach up to 50 meters into the ground.
https://www.deutschebahn.com/de/presse/pressestart_zentrales_uebersicht/Planung-fuer-Fernbahntunnel-startet-Taeglich-250-Zuege-mehr-zum-Hauptbahnhof-Frankfurt-6263398https://www.fernbahntunnel-frankfurt.de/home.html 