The MATE desktop that comes with Ubuntu is very amenable to customization. One cool theme idea is to try and make it look like Windows. There are Windows Vista icons that are very nice. They can improve the look of your desktop a lot.
Download them here: https://www.securitronlinux.com/linux/files/Vista.tar.gz.
That would be a great icon theme for a Gnome or MATE user.
Make MATE or Cinnamon desktop look like Windows 10.
https://securitronlinux.com/debian-testing/make-the-cinnamon-desktop-look-like-windows-10/.
The themes mentioned in that post are awesome and really do look like Windows 10.
I just wish there would be a Windows 11 theme for Gnome and MATE. That would be cool. There is an icon theme of Windows 11, download it here: https://store.kde.org/p/1546069/. This is great, it fits the MATE desktop perfectly.
Old Ubuntu Human theme for GTK 3 and 4x.
https://www.gnome-look.org/s/Gnome/p/1376363.
This would be cool if you are a fan of the old Ubuntu look and want to use it on 20.04.
Professional GTK theme for MATE and Gnome.
https://www.gnome-look.org/s/Gnome/p/1334194.
This is a very clean and brightly coloured theme for professional use.
Classic Ubuntu wallpaper archive.
https://ubuntu.ecchi.ca/wallpapers/.
This is a good archive of all classic Ubuntu wallpapers from years gone by. This is very interesting.