In my experience of only a handful of rifles, customizing takes two paths.
1) To fix or improve existing faults or design compromises.
2) Personalising your rifle to taste.
Often the two blend together into a unique rifle. Done well and looked after they can add some value to a rifle. If to0 unique, it will make the rifle too specialised to retain value.

Here are few examples of custom work that can be done.

Stocks can be modified to fit you better. Length of pull shortened or lengthened. Weight added or removed. Shape of the pistol grip changed to fit your hand. The action can be bedded (made to fit better) for more consistant accuracy.

Sights can be removed, changed or improved for a better sight picture. Peep and scope sights added. Mounts changed for weight or height. Recknagel and NECG make and sell a variety of options for CZ's.

Pistol grips and floorplates as well as the whole rifle can be engraved for a personal touch or to restore a fond hunting moment/memory

If there is nothing wrong, don't fix it. Look for small improvements if needed.