+1 But in this case need to teach the player character to rotate the head inside vehicles

Yest but only if they select fully interior making. I was thinking about editing existing image of periscope to add 2 on both sides and changing zoom power. Maybe use PiP but i prefer standard soluton (PIP can be very laggy). Of course 3d interior only for driver is very good solution - They dont need to create fully detailed interior - just periscope level - rest of "interior" can be darked in shadows or blurred, and player will be not allowed to look down and up too much, zoom power can allow to look only at 2 view ports at same time, to look through 3rd you need to rotate head. There is so much option which doesnt require much work, so i think BIS will select best for us and for them.

I think every armored vehicle in the Arma3 needs interior modelling also. Because all vehicles in the game have the interior!
(except armored vehicles)

Remember OFP? All vehicles had interior. But imagine how low performance you will get? 10 fps less, depend of how many vehicles on the map - note that this is not OFP - this is deferent engine, better graphic, other lighting, EVERYTHING IS DIFERENT. I prefer minimal interior level, but with ability to at least move your head in some dark are to look through view ports (only them are visible - rest in dark). Also does anybody use interior? This will be useless, gunner need to look through optic, commander too. So only driver and passangers should use interior

It makes sense for vehicles to have an interior, Vlad.

For it allows the TC to do more than just stare out the optics, he can look at things like the IVIS which gives him a better situational awareness.
Also looking through an CITV/Peri is still like looking through a straw compared to using the vision blocks in the commander's cupola.
As for the gunner, well not as important but it could provide immersion, by having a non-hud way of showing weapon states.

As for the Performance, well the bradley in Arma 2 had one and the performance wasn't bad.

Aside from that, look at the new BW Mod, they've made some good gunner, driver and commander's position.
For the driver they made a quick and dirty solution by just adding additional viewports in that mask.

Also look at the old tank simulators of the nineties, their interior was 2D often clickable, but it worked.
It didn't require a lot of performance but gave you a better situational awareness and immersion.

OK, its up to them how they do that.