Increasing chassis stiffness?

Hi. I would just to ask if what parts/braces are highly recommended to increase a car's chassis stiffness?

Car is a 2015 Honda City.

Added (1). * The Honda City that I'm talking about are the ones that are produced in Honda's Asian markets.

Self driving cars should be mandatory.

Why would you want to increase chassis stiffness?
this is either required for driving very fast in a competitive manner or for carrying heavy loads. Your car is the wrong car to do this.

if you want to drive fast. The way to cornering performance is tires first. Then reduce weight. After that you can upgrade suspension and start dialing in proper ride height, spring stiffness and damper settings for bound and rebound.
after all of this if you are noticing your car is inconsistent with the way it behaves after setting changes then you improve the chassis stiffness.

in this case you should look for an engine bay strut bar and some custom fabricated chassis mods.
also check any solder points and if anything is tack/spot welded, have it redone as a full line weld. This eliminates flex in the joints of the chassis.

for carrying weight i'm guessing its a similar process but with a higher requirement to improve the brakes, cooling especially for the gearbox and springs to keep the car from bottoming out under the weight.

for both of these cases, you are in a very bad position because your car is not meant to go fast nor carry heavy loads. It is easier and cheaper to buy either a car that is faster or a car that can carry more.