So I was hoping to get some insight on this topic. I was overlooking the guilds in Ravnica and wanted to implement them into a world but I was wondering what should I do if all the players don't choose one guild. How would other guilds like their members going off on opposing guild missions?

I found that the guilds are something to bring up in a session 0, as they can warp how the game will be played. Or they did in my two Ravnica campaigns.

The first had everyone from a different guild, I engineered a fairly flimsy agreement between the guilds that the players were in to investigate a golgari plot which led to this special taskforce being ratified by the guildpact. I guess the problem that I encountered was after about 25 sessions I was struggling to keep things interesting. I had run though a couple of the more morally dubious guilds but it followed this pattern where the players would hear about a possible threat, investigate the situation, then try to diffuse/destroy the threat. The game ended after they finished dealing with a splinter group of simic who were sinking part of the city into one of the seas. It felt like a high point to end the campaign and whilst it could have continued, the group would have been dealing with an azorious crackdown using the previous events as cause, but it felt a good point to stop. I found at times the group was being pulled apart with their various allegiances and others it produced some really good roleplaying. Its was just a bit too hit and miss. Also nearly everyone had asked to play an ozhov enforcer/preacher if their character had died. So I floated the idea of starting a new game.

One thing I would mention is that you will want a plan dealing with renown as some of the rewards put the players in charge of large numbers of soldiers, knights, mounts etc. I found it forced me to think of widespread threats where the players could put them to use rather than just ignore them.

When we talked about the ozhov it was clear that the group didn’t necessarily want a long campaign for the second one, so we decided that they would all be working together to undermine azorious lawmaking. It also allowed me to recycle some of the content that I devised previously. This ran about 10/11 sessions in all they also had a little rivalry to see who could gain the most wealth in the time frame.

I see what you're saying but my question was what if the party decides to pick different guilds. Is that just not an option and the party needs to choose one?

It's fine to have them from different guilds, but you will need to have a plan for why they are adventuring together and I would discuss this in your session 0. You will also want a plan for tier 3 and onwards as by that point they will wield a fair amount of power and responsibility within their guilds.

The start of Chapter 1 gives a few good ideas but most are if they are all part of the same guild, the party makeup section (also chapter 1) is a great starting point.

Here are a couple more generic ideas if you have players from more contrasting guilds

• They could have all been friends before they joined their individual guilds and a some event brings them back together could be a funeral of a mutual friend so they decide to investigate their death.
• They were tasked to investigate a rise in worship of Nephilim and each guild has sent a representative (mainly to make sure that it's no-one from their guild behind it).