1. efibootmgr is already part of the script (no effort)
2. Have the grub package included (very little effort)
3. Offer the option to use UEFI (very little effort)
4. For advanced user experience do a post-check on the layout after choosing this option to warn the user of possible misconfigurations (some effort)
   (store the variables globally for later access):
   • Check for the partition table type (store and compare to string):

     DISK=`lsblk -o MOUNTPOINT,NAME | grep "/mnt" | tail -c 5 | sed s/.$//` && TABLE=`sfdisk -l /dev/$DISK | grep "Disklabel" | sed 's/Disklabel\ type:\ //'`

   • And throw a warning if its not GPT (Cannot simply evaluate true to msdos as there are many others [script must not exit here, user might wanna run UEFI on MBR]):

     [[ $TABLE != "gpt" ]] && echo "Warning: Your disk was not formatted with a GUID Partition Table! \nTo boot with UEFI it is highly recommended to use a GPT table."

   • Check for the ESP (store and compare to false):

     ESP=`lsblk -p -o PARTTYPE /dev/$DISK | grep 'c12a7328-f81f-11d2-ba4b-00a0c93ec93b'`

   • Throw an error if it doesn't exist (optionally exit out of the script, need to evaluate the user experience at this point [Thinking about it rather make it a prompt whether to exit out of the script since the user might decide to uncheck the option/use default BIOS instead]):

     [[ ! $ESP ]] && echo "Error: No EFI system partition found! \nManually check your disk layout before continuing." && echo -n "Exit the script to review partitions? "; read CHOICE

   • Else just keep going

if [[ $FIRMWARE = "UEFI" ]]; then
  echo "You can choose one of these bootloaders: " 
  Create your recent up/down selection menu (PS3 select?) with GRUB/EFISTUB instead of Yes/No
  if [[ $GRUB ]]; then
    install GRUB (see below)
  else
    install EFISTUB (see below)
else
  install syslinux
fi

• Get the ESPs mount point (This should be /efi for GRUB as outlined in the Wiki but the user might not care and use the deprecated /boot instead):

  EFIMOUNT=`lsblk -p -o PARTTYPE,MOUNTPOINT /dev/$DISK | grep 'c12a7328-f81f-11d2-ba4b-00a0c93ec93b' | sed 's/c12a7328-f81f-11d2-ba4b-00a0c93ec93b //'`

• Chroot into /mnt (Use `arch-chroot`!!!!!) to install and configure Grub. Other chroots do NOT work without bugs/nuisances/errors at this point (Unsure whether the call to the variable would work in this context, might become local to the chroot environment?)!

  arch-chroot /mnt grub-install --target=x86_64-efi --efi-directory=$EFIMOUNT --bootloader-id=grub && grub-mkconfig -o /boot/grub/grub.cfg

• Done. Continue script

• Check the ESPs mount point (This should be /boot for the script to work. Need to error out if it's not [because it's possible but needs customization of the --loader option]):

  EFIMOUNT=`lsblk -p -o PARTTYPE,MOUNTPOINT /dev/$DISK | grep 'c12a7328-f81f-11d2-ba4b-00a0c93ec93b' | sed 's/c12a7328-f81f-11d2-ba4b-00a0c93ec93b //'`
  [[ ! $EFIMOUNT = "/boot" ]] && echo "Error: Your EFI system partition is not mounted at /boot! \nThis script needs it to be mounted at that location. \nExit the script now?"; read CHOICE

• Get several more system details and store as variable (only locally as they'll not be need elsewhere):

  PARTITION=`lsblk -p -o PARTTYPE,NAME $DISK | grep 'c12a7328-f81f-11d2-ba4b-00a0c93ec93b' | tail -c 2`
  ROOT=`lsblk -p -o MOUNTPOINT,PARTUUID $DISK | grep "/mnt" | tail -c 37`

  (the tail command counts line breaks so we need to cut off what we want + 1)
• Make the NVRAM entry:

  efibootmgr -c -d $DISK -p $PARTITION -L 'Obarun' -l \vmlinuz-linux -u 'root=PARTUUID='$ROOT' ro initrd=\initramfs-linux.img'

  (Make sure the single quotes are correct! the --unicode option is a string, so it needs to be escaped right for variables to be used)
• Inform the user about manual intervention:

  echo "Notice: Microcode and custom kernel parameters require manual reconstruction of the NVRAM entry. \nRefer to https://wiki.archlinux.org/index.php/EFISTUB# efibootmgr and efibootmgr(8)"

• Done. Continue script