Is there a hotkey for increasing and decreasing the tab stop value? If not, I think it would be nice to have.
You want to change the tab stop value on the fly? Why?
Do you know that it is possible to define tab stop value(s) per file extension?
No, take a look at File extension based word wrap, tab and indent settings.
Or do you edit tab aligned assembler files or CSV files using tab character as separator?
Yes, do you know that multiple tab stop values can be defined for individual tab widths?
No, open Advanced - Settings/Configuration - Editor - Word Wrap/Tab Settings and click on button Help. It can be read in the opened help page that up to 12 tab stop values separated by commas can be defined for all files with a file extension according to currently displayed list item at top of the configuration dialog.
But to answer your question: The tab stop value can't be modified by hotkey, macro or script. Configuration settings can be modified only by the user via configuration dialog.
Of course it is possible to increase and decrease the indentation of selected lines according to tab/indent settings as defined in configuration for active file by pressing TAB and Shift+TAB after having made the selection.