Back in the days of tabletop roleplaying, my friends and I always had this standing rule: If you can explain your concept so well that it could fit into a game manual and not look out of place, have at it.
This means, if someone wanted to play a necromancer character in a game that did not allow for players to technically be necromancers, they would have to first of all, build from an existing job class and second of all, explain WHY, out of all the millions/billions of people in this realm that aren't necromancers, they are the unique snowflake of the bunch.
Sometimes people could pull it off, and we would end up with a player-character that was unique and interesting and really fun to play with. But most of the time it was just someone who thought they had found an "easy" way to play something awesome, but hadn't really thought it all through.
In an MMO, it would be even harder to integrate because you don't have a personal GM standing around to say "Oh ok, sure we can change ____ spell to become a necromancer talent." The endeavor would rely a lot more on the group's willingness to pretend.
In all honesty, when RPers tried things like that in XI, it tended to lead to poorly played god-moding. One common example was the unstable, dark and brooding but very strong loner (usually a dark knight). I can't count how many times one of them would threaten to murder another player character in game, or insist that the player characters weren't safe around them in the city. Not only was that completely impossible to be true (we can't PVP in cities in XI, and even if we were to collectively pretend PVP anywhere other than a battlefield game was possible, there are still armed guards everywhere...), but it came off as improbable (why would so many lonely DRKs join a linkshell just to tell everyone to go away or they'd murder them?).
So getting back to the necromancer example... my short answer is that it would entirely depend on how the player approached it. If they saw NPC necros like in the TOAU expansions and said "hey that would be a cool theme" and just wanted it for flavor text (as in, I blow up stuff with black mage spells on weekdays but in my spare time at home I re-animate neighborhood pets) that would be more plausible than someone who said, "I am a necromancer now and my heart is filled with the icy evil of UNDEATH! Tremble before me or I shall raise a skeleton army against San d'Oria!"
This means, if someone wanted to play a necromancer character in a game that did not allow for players to technically be necromancers, they would have to first of all, build from an existing job class and second of all, explain WHY, out of all the millions/billions of people in this realm that aren't necromancers, they are the unique snowflake of the bunch.
Sometimes people could pull it off, and we would end up with a player-character that was unique and interesting and really fun to play with. But most of the time it was just someone who thought they had found an "easy" way to play something awesome, but hadn't really thought it all through.
In an MMO, it would be even harder to integrate because you don't have a personal GM standing around to say "Oh ok, sure we can change ____ spell to become a necromancer talent." The endeavor would rely a lot more on the group's willingness to pretend.
In all honesty, when RPers tried things like that in XI, it tended to lead to poorly played god-moding. One common example was the unstable, dark and brooding but very strong loner (usually a dark knight). I can't count how many times one of them would threaten to murder another player character in game, or insist that the player characters weren't safe around them in the city. Not only was that completely impossible to be true (we can't PVP in cities in XI, and even if we were to collectively pretend PVP anywhere other than a battlefield game was possible, there are still armed guards everywhere...), but it came off as improbable (why would so many lonely DRKs join a linkshell just to tell everyone to go away or they'd murder them?).
So getting back to the necromancer example... my short answer is that it would entirely depend on how the player approached it. If they saw NPC necros like in the TOAU expansions and said "hey that would be a cool theme" and just wanted it for flavor text (as in, I blow up stuff with black mage spells on weekdays but in my spare time at home I re-animate neighborhood pets) that would be more plausible than someone who said, "I am a necromancer now and my heart is filled with the icy evil of UNDEATH! Tremble before me or I shall raise a skeleton army against San d'Oria!"
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The Adventure League of Eorzea: A progression & RP guild. FFXIV's longest-running RPLS