No, that is not possible. Arnor and Gondor would need to have the same spellbook, so we would have to remove all the spells fitting only for one of them.
Regarding Gondor that are the following spells: Army of the Death, the Rohirrim spell, Woods of Ithilien, Gandalf the White, the fiefdoms, the Twins + the Rangers, which are most of the cool spells Gondor has.
Also the AI does not accept this, you can already see in 3.81 that the dwarven AI uses all the heroes, which would be much weirder when playing Gondor. Besides the fact that there is a maximum number of heroes for a faction.
Furthermore, what should be the name of this faction? We can't use Men, because the Rohirrim are Men, too.
Arnor will stay a map feature of Gondor.