To avoid any possible democracy vs monarchy, Royalist vs Orléanist vs Bonapartist, empire vs. kingdom arguments breaking out, I've purposely not specified a kingdom or an empire.
Whether you take as starting point 1793/1814/1830 (constitutional monarchy) or 1804 (empire), how would France have developed along monarchical lines if it had continued in the form of government (royal/imperial) to the present day (where it is a constitutional monarchy a la Britain), rather than going through several different forms (a directory, three kingdoms, two empires, one “State” and a grand total of five different republic). And what things might be different in French history aside from having a king/emperor?