I have a game theory where the difference between player numbers rank the same level of difference between dueling and 3 player, to then going to 4 players, and so forth.
We lump the term "multiplayer" to categorize anything that is not dueling, but the dynamics between number of players is just as dramatic a change between dueling to multiplayer, and it's a topic that I haven't seen at all in threads.
Now where as 3 player is still skewed towards early advantages still, a 5 player game is often about attrition. Almost certainly the first player who tries to "combo" off is going to be invariably met with ways to be stopped. After all you have 4 other players at the ready.
Now especially with planeswalkers incidental damage from the other 4 players creatures is also a lot harder to get traction.
The answer is that it's much more about building up to a threshold of mana and to pick a spot where you combo off in one big turn (likely a row of turns with the extra turn cards).
What I mean is that it's unlikely that you stick an early Will Kenrith and Rowan Kenrith for example and are able to ride them out for an extended period of time.
So it's more of a patience game and dealing with the worst your opponents are doing and trying to find the right spot to then try and do a lot of things in a sudden burst.
Most of my games are 4 player. So this deck is more or less tuned for that.
The first thing to change for say a 5 player game, is to reduce the number of planeswalkers, as simply put they are less likely to gain you continuous value.
On the same note with higher number of players, if it's not your planeswalkers that are being attacked, then it's your life total. This is a very creature light deck and your opponents will learn to target you as you'll be far less likely to have blockers. The thing is that opponents don't need to commit that much to the board. If you have 4 other players who put 4 power for example say each, then you are potentially being attacked for 16 damage each turn if they decide that you are the most likely to remove them.
So where as I'm definitely not normally a fan of defensive cards like Propaganda, in a 5 player setting it can really help to stall out the game long enough for you to setup the big turns that you will need.
Setting up the big turn(s) requires having enough mana to do a number of things. PLUS you want to only do this at a stage that you feel or know opponents are not holding up too much disruption. This is where patience is needed and often wait for another player to have tried a big sequence of plays to have the other players firing off their counterspell and removal, etc. OR simply when you get most opponents tapped out.
There are a few things you can do to achieve this. Being able to cast more things at instant speed, means that you can do a number of things before your turn freeing up mana.
Leyline of Anticipation and Emergence Zone are the best candidates for doing this.
The idea is that you can setup a lot easier by casting planeswalkers before your turn, so that in your turn you can cast a lot more cards.
Now the more players there are, the more of an attrition battle it becomes, so having more disruption can be beneficial as well. There is a lot of mass creature removal in the deck, but it's far more likely that your opponents bounce back because often players are less likely to commit too much to the board knowing that it's more likely that there is going to be mass removal several times during the game.
It's also more likely that there are more counterspells floating around at any stage of the game, and unfortunately (or fortunately depending on your point of view) it's beneficial to have more of these in the deck for yourself.
All of this is meta dependent as well, if you have a more competitive environment then counterspells become a lot more necessary, but if your playgroup is quite creature attacking focused, then you are better off having more defensive cards.
But here goes these are the sorts of changes I would look to make for a 5+ player groups.