I would like to have a possibility to have custom dynamic field which has static part (e.g. [y-x]) dynamic string value depended on other field (y) (selectbox or other) and autoincrementing value (x).

The idea behind this is requirements management, lets say you have tracker Requirements, and you have two sub-systems: SYS1 and SYS2. Your requirements could look like [SYS1-1], [SYS1-2], [SYS2-1], [SYS2-2], [SYS1-3] and so on. In the tracker you can have selectbox with SYS1 and SYS2, so selecting it, the custom dynamic field should use the next integer value for that selectbox item (SYS1, SYS2, etc.). The dynamic field settings should let you specify the static format part ("[y-x]").