IFV, APC, and MBT crew should be able to open hatches and stick out.
So the title says a lot of what my suggestion is:
Crewman in the driver/commander/gunner seats should be able to open the hatch, stick out, and observe from that position. Commander roles should also be able to use binoculars in this position as well. On top of that, people in the driver positions should have the ability to scroll to zoom into the viewing slits they are sitting in front of as well.
I know some of you may not like this suggestion, especially because it may bias the balance towards the already powerful vehicles, but I disagree. Especially considering that if someone goes into this position they are vulnerable to small arms fire. You're increasing the chance of viewing enemies while also increasing the risk to the crew drastically. For example, a tank driving down the road with the driver and commander sticking out may have a lot better visibility, but you can also basically wipe out the crew with just small arms fire.
For example the Red Orchestra series of tank combat is exactly what I am talking about. I cant post a link, since I am just now joining the forums, but please search on YouTube "Red Orchestra 2 tank combat."