I recommended none because the stock configuration is very good. You have good airflow with air going through the front and it being drawn up, going through your hardware and leaving the top or at the rear.
If you add a bottom intake, then your front intake is useless. The bottom intake will act as a wall so that air from the front intake is pushed upward or blown off course. That means that your GPU will not get sufficient airflow. If you make the bottom fan as exhaust, then your front intake is useless with all air blown in going right back out and the bottom exhaust is fighting against your top exhaust. This would render less air in your case.
Top intake would be fighting against the top exhaust and mother nature. As hot air naturally rises, it is not recommended that you push it back down. This also clashes with your other top exhaust. Another top exhaust will result in pushing out more air than you can pull in. So that results in poor airflow as well.
Side panel is okay, but not necessary if you have to ask. If you need a side panel intake, you would know the reason for it.
Point is, adding more fan does not always help. More fans = more air, but if they are not guided and are blowing in random directions, the only thing you end up doing is causing more turbulence. This results in poor airflow. And most fans have a lot of dead air space, so there will be many more dead air space for more fans you add. Think about it. Wind is best run at one direction. Because you can ride it or direct it easier. If you have air blowing at you at all directions, it doesn't help you. It's actually more annoying.
Also, air is a poor heat conductor. So it doesn't cool well. Otherwise, your pot of soup will be cold very quickly.