Not sure if it is possible or not but being able to update the bootloader from the GUI would be an awesome add. Obviously this can be done from the shell as is well documented around here but just an idea for those not wanting to run shell commands.
Not going to happen : https://forum.opnsense.org/index.php?topic=52134.msg268594#msg268594 :)
As long as you boot in UEFI mode it's basically two simple commands that do the trick without any risk of writing to the wrong partition...
I did my first ever Bootloader upgrade this week and after reading a couple of topics here it was pretty easy!