For the first question
-Removable hdd and drive blocks can be good. So that the case can support more aio coolers on front and case will perform better for air cooling.
-What i cant do with haf-x is i cant use 200mm noctua fans on the top. Because they are not round shape like your mega fans they are square shape ones. Square 200mm fan support on the top can also be good.
-On the bottom of the case near the power supply, we can get another fan place for intake.
-Better cable management to hide cables on the back of the motherboard and the count of holes for the cable management can be increased and put around little more for better cable hiding.
-For airflow focused case, we can get small fan controller adapter that will be plugged in motherboard so that we can manage to change rpms of all fans from motherboard software when cpu or gpu temperatures increases too much. Sometimes you get too much fans or a few fan pins on the motherboard therefore you have no space for fan pins on motherboards so you put the fan directly to the power supply with static rpm. Fan controller can also be good however it will increase the cost a bit.
-For memory cooling, we can get another fan on the right side of the panel directly towards to memories so that we dont need a small too noisy memory cooler for overclocking(I prefer at least 120mm or 140mm for silent cooling). haf x vga bracket is a good example for this. But that bracet should be a small and least plastic on it possible so that intake fan on front doesn't effect badly for case cooling.
-Lastly, For aio cooling solution on the top we need more variants of cooling types like ( As i know haf-x supports most of them but i want to give you an idea about variations. It is better you get advice from more experienced users that use more aio cooling on haf-x than me but i can list some as i know.).
aio cooling cpu 360mm-480mm (top or front) with aio cooling gpu(s) 240mm (top or front).
air cooling cpu with aio cooling 360mm- 480mm gpu(s)(compatibilities with big air cooling solutions)
-About the look of the case, it can be better to have simple look than robotic one like haf-x. It is not that much a problem for me but some users like simple look cases.
-We can get a decent front panel with usb-c, usb 3.1 etc.
-Instead of the screws for the panels, we can get sleeve ones so case will be dismantled easier. Another issue here is you cant remove front panel without removing top panel that is an annoying thing to be honest. Panel dismantling is also another issue it can be done easier without using the clips inside the case.
-For rgb, as long as it doesn't affect case cooling performance it is not a problem for me. However if it affects cooling performance then it is better for me not to have rgb on the case.
-About glasses on the case, it is better to have meshed and filtered case to have better air cooling in it. As i know in the h500m you can't remove top glass for the meshed one.
-The dust is another big issue in the case. If we can get removable dust filters on the all fans, we can clean the case with no time. At the moment you need to dismantle the case and use a compressor to clean the case well.
For the second question
- I prefer the haf-x sized case because it is easy to build my case without worrying which part to put in the case first.However If we can get another case like haf-xm series that give you the smaller size of the new haf-x will be better for some users who lack space etc.
Thats all i remember right now. I hope it will help you to build better haf-x.
Thank you and have a good day.