OMG FINALLY A POST I AM QUALIFIED FOR, I am on my 9th case in 2 years and so let me speak.
If you like the look of the cosmos but want a side window I really recommend the Cooler Master MasterCase Pro 5 its about $300 sold by PB mainly. It has that same CM design language as the cosmos and has the side window.
Its an amazing case for cooling with support for 140mm fans in all of its slots or support for 240mm rads in front or top. If you are wanting to optimize cooling the 5.25 bay can be removed as well as the HDD bay and you can create a un-obstructed wind tunnel. This will still leave you with 2 HDD rack in the bottom compartment and 2 SSD slots either on the rear or on top of the shroud divider. Bear in mind with high airflow is high dust build up but the case was super easy to clean out.
Another case with good airflow with a side window would be the Corsair Air 540 or 240 pictured in this thread. Though this case has been reviewed as being flimsy in build quality (see Hardware Cunucks ) and of course its a 2 compartment cube so some parts don't fit. Not to mention the 340 only takes 80mm fans in the back and unless you want to buy really expensive 80mm fans from Noctua don't bother 80mm fans are loud.
Fractal design makes great cases that fit your criteria. The Define series has been refined over 4 years now and its really a delight to build in. Just like the Mastercase, the Define R5, you will get a side window with a view of your gear and the cases are great for airflow. The only down side is that divided cases are 'in' at the moment (and my pref) and the Fractal cases don't have em so your guna see your PSU through the window and the nest of cables. Alternatively on these same lines the Corsair 350d and 750d are in the same field as the Define R5
Avoid NZXT if your wanting airflow. The S340 is a CHEAP case brilliant case to work in but the very definition of cheap and if your building big you dont want to cheap. the H440 is an amazing case but as its more focuses on silence the airflow suffers, this is my current case and while my parts are not overheating ambient temperature is higher than when i was in my Mastercase and it takes longer to vent.
Phanteks Cases are great, they often come with a 200mm fan in the front which is great for airflow. I would avoid the older EVOLV models they all had 1 thing which wasn't quite right but the late 2015 model really hit the nail on the head. Also very fun to build in. But the Enthoo cases are very elegant and a good choice for a high end build - The deluxe comes with built in LEDs.