There's a very large plot of green land in Ciyoda, right in the middle of the city . . . . but I doubt they'll move the emperor.
There are quite a few rather good sports facilities already, quite a few of which will be used. They're building a new main stadium - but it'll replace the one used for the 1964 Olympics. The same site also houses other sports facilities which will be used.
Accommodation for athletes will be on another site on Tokyo Bay, which is being redeveloped. Post-2020, it'll be sold off as new housing. It's convenient for the city centre & easy to get to the 1964 site from. There'll also be new sports facilities there.
To quote the IOC summary -
There is no risk attached to land acquisition as all land required for the venues has already been secured. ...
Of the proposed 36 competition venues:
• 15 exist. 13 are in excellent condition and two would require permanent works;
• 10 venues would be temporary and dismantled after the Games;
• 9 venues would be new, permanent Games-dependent construction;
• 2 venues are planned irrespective of the Games, including the new Olympic Stadium to be constructed by the Japanese Government through the Japanese Sports Council.
