Hello,

I am about to start my first campaign as a DM and I am not sure if the behavior of on of my players is going to be a problem. He is a a big mtg fan and originally wanted to play a vedalke form a kaladesh, a plane outside the traditional cosmology. We debated about it and after a while I had to tell him, that I wouldn't let a level 2 character alter the cosmology of my world or have any knowledge of the outer planes.

I originally wanted to try and enable him to play the unearthed arcana race by integrating it into my world in a down to earth way. After a lot of debate over the nature of planes and magic he told me, that he didn't want to play it, if he couldn't play it with his world altering fluff he wants to play something else. Afterward it seemed like a lot of wasted time for both of us because I tried not to forbid his wished race. Did I in your opinion behave correctly here or should I have prohibited it right away? What would you have done differently?

This leads me to where I am now:

His new character is a Triton paladin, how after a short Google search is identical to a existing character from mtg. He has the same name, story and probably abilities as him. From my understanding he does not want to use any of the existing Triton lore, but wants to use the lore of some mtg race and location. I think this could be a problem because he does know more about the mtg lore than me and just assumes that things work the same in my world. This also causes problems in integrating his backstory into my world. The other thing I am worried about is, that he might be restricted in his decision making by using a preexisting character. I haven't talked about this with him as I wanted to get some advice before proceeding.

What would you do in my situation? I don't want to curb his enthusiasm, but I don't want to waste our time again by trying to integrate his character. Do you have any advice how to deal with the whole preexisting character thing? (this is not an character inspired by fiction, but more of a carbon copy of a fictional character) 

Any advice is greatly appreciated

What mtg character is he copying?

Suggest he takes the outlander background, have him be not sure how he got to this plane or any idea how to travel planes.

Alternatively, you can try to work in his character's home in as a distant land native to this plane. As long as he doesn't try to use his mtg backstory to gain an advantage it should be fine.

You don't say how new the player is to DnD. It sounds like they are fairly new to it.

On the one hand, dont be too critical of their choice to copy some mtg content. It's where they are comfortable starting, and must sound like a fun time to them.

So I don't think you wasted any time trying to accommodate them the first time around.

It sounds like you have to have a conversation about game balance with them, just explain the game involves both storytelling and some mechanical rules. You want the player to have as much freedom as possible on the story front, but they don't get that same freedom on the mechanics side.

To make things easier on both of you, at that point you can let them know, they need to pick from existing options for a their mechical choices but it can all be resigned, story-wise.

So if you pick Triton as a race, you get exactly the abilities and benefits of the Triton race. You can call it something different, change the descriptions, visuals, the 'fluf'. But none of those changes makes any difference at all mechnically.

Let him come up with as much detail about his character's race, background, etc as he wants to, with the guiding principal of, "it doesn't change the stats".

As DxJxC suggested,  an Outlander background is perfect. Plenty of ways to travel one way. Being from some other plane doesn't mean the character understands the planes, or knows how to travel them on their own.

In fact the character may believe they understand many things about the cosmos, only to find out they are totally wrong.

I had a player, new to the game, that wanted to play a super hero, very literally. We sat down and talked, he described the types of things he imagined the character doing. We decided to use monk, and for his character,  it's not training and discipline that give him these powers, it's part of some secret family tree backstory stuff. He's ended up getting most but not all of the effects he wanted out of it, and was super happy. I got lots of ideas for custom magic items for the character out of the things he wanted to be able to do that monk didn't cover. Worked out well.

The player isn't new to dnd, he was our previous DM(I believe he doesn't have a lot experience as a player) and the problem I see is not mechanical. I am afraid that he will limit my agency as DM, because he knows a lot more than me about the mtg lore and sometimes behaves as though I should know the same. I don't. His character is a Triton paladin named Kumena and comes from a sunken city named Orazca. Without much google-fu you get here: https://mtg.gamepedia.com/Kumena

I am afraid that, if I ever describe something linked to his backstory, like orazca he will go: "no it's like this:" or that there will be problems cause his character is pre-existing 

I am getting ready to start a campaign/world from scratch.  I am not inexperienced in D&D.  The rest of my group is decidedly experienced, not only, in D&D but fantasy as a whole.  More so than me.  So, I am doing homework.  I am reading Dragons of Autumn.  Primarily because my brother wants to make a character based on Raistlyn.  I asked him what the character would boil down to.  He told me that he had the staff and eventually wanted to defeat a god.  Another told me what they wanted to do.  It boiled down to Level 20 dreams on a Level 1 character sheet.

I began my response to each friend's request with a NO.  Then it became a, well, bring it to session zero and we will see what we can do to make it fit.  Realize that you might be able to end up doing all the cool stuff you want to end up doing.  but it is going to take a LONG time to get to that point.  It is going to take a lot of play and work.

And maybe this is the tack you need to take.  As to background and such, it has to at least be able to be made to fit the narrative of your story.  If it does not, tell him it does not but come to that conversation armed with GOOD alternatives that get your friend close.  Let him know he might be able to alter the cosmos at Level 20, but he cannot do that on a Level 1 character sheet.  It is going to take time.  Let him have this in mind.  And as his player grows into your world, he might find his expectations shift a little and something else completely unexpected might just come down the pike.

Usually when it comes to PC, I have my players use the Matt Colville approach to character creation. 

When they come to my table, I have them roll the Ability Scores (4d6, take the lowest) but they're done in order. I.e. their first roll is for STR, then for DEX and so on. That way, they don't bring any baggage to the table and they "discover" their character for the adventure.

If those characters die, they can roll the normal way, but the first way helps me by not having to force their "square" character into my "circular" world.

If you've already started, then I agree with Conn_Eremon. Let your player know he is not playing Kumena of Orazca, but a re-imagining or someone who looks up to Kumena. 

This isn't helpful, but why kumena? there are way cooler mtg characters to plagiarize