Proximity radar should be enabled by default, just as you did in the days of proximity arrows.
At the time users who do not follow the forum, our discussions or read the changelog will not even be informed of the new feature and in fact makes proximity radar useless 
For developers: please consider adding an opaque background or the option to select the opacity of the radar background.
Also,I think radar should always on hud,or make it an optional option. I noticed that if a car’s relative position to you remains unchanged for a short period of time, the radar will automatically turn off, even if the following car is only 0.2 seconds away from you
It should be more sensitive and perhaps have a slightly matt background to stand out better, especially in races in the night.
Yes please, have it enabled by default for Multiplayer at the very least.
Any tools/settings in the game that can assist with “cleaning up” online racing should be enabled by default in my opinion.
Forza Team ignored this request on Update 11
As of Update 20, Proximity Radar is enabled by default in Featured Multiplayer:
Players can still adjust its placement or disable the feature via the Settings menu.
This topic reached 50 votes in Motorsport Features > Menus & Interface