look into omnirom...
I am not going to say any ROM is better than others because all ROMs have their pros and cons. To find out which ROM you like best you will have to try and experiment for yourself.
Nexus devices have a wonderful recovery manager which is TWRP which stands for Team Win Recovery Project. Nexus devices also have a great tool which works with TWRP recovery manager, this tool is call MultiROM manager. What multirom manager does is it allows you to install multiple ROM's on your Nexus device without harming or altering your stock ROM.
In order to use multirom you need to first patch your stock ROM with the kexec-hardboot patch. That can be done very easily. First thing you need to do is root your device. If you are going to root your device i would also recommend unlocking the tablets bootloader. This can all be done with WuGFresh's NRT (nexus root toolkit). Here is a link for NRT -> http://www.wugfresh.com/nrt/
After you have your tablet rooted and bootloader unlocked then install TWRP recovery, again all of this can be done with the Nexus Root Toolkit.
After that you can go to the google play store and download the app MultiROM Manager. This is the easiest way to install multirom. Here is the link for multirom manager -> https://play.google.com/store/apps/d...ar.multirommgr
Again, if you don't have your stock kernel patched with the kexec-harboot patch you won't be able to boot multiple roms. You can patch your stock kernel with the kexec-harboot patch from multirom manager.
So once you have unlocked bootloader + root + twrp recovery manager + kexec harboot patch for stock kernel (from stock rom) + multirom manager you have the freedom to do whatever you want to your tablet without harming or modifying your stock rom.
As for a custom ROM i would recommend that you look into OmniROM.
It has gotten good reviews and i personally love it. I find it very stable and it also has a really nice feature with multi-windows and split screens.
Another thing you might be interested in is look into "xposed framework"
that allows you to customize your stock or custom rom by adding things like multi-windows or advanced reboot menu for example.
I hope this has been helpful...
If you get stuck on anything there is great documentation on rooting + twrp recovery manager here on xda or you can find great videos on youtube.com.