Knowing your players, if you had a year of free time to prep as much as you wanted for a campaign, what would you prep? Would you build an entire world? Focus on making custom mechanics? Homebrew subclasses? Would you use the entire year? Where would you start?

I'd spend a week reading The Dark of Hot Springs Island... and spend the rest of the time drinking on the beach at Ho'okena.
You can run it straight from the book if you are familiar/comfortable with nested tables (I'd use chartopia for the ease of it).

Or Barrowmaze for 5e. I ran that using LotFP rules once having only skimmed the book on a flight.

I'd run the campaign I'm running right now... I'd just have multiple dungeons prepared ahead of time instead of 1 or 2.

Easy. I wouldn't. Because, as the old Dungeon Master saying goes: "No plan survives contact with the players" and with mine that is often more true than not.

I've literally had this hypothetical manifest into reality, what with the pandemic closing my workplace down for most of the year, so I can answer this by saying I finally came up with a well fleshed out world setting, with an exhaustive detail in culture, customs, unique laws, and webs of politics. My players are really great, and trusted me that it was worth reading, and now we're playing in the world with characters who all have detailed backstories. 

I've also been 'novelising' each session afterwards for everyone, but I dunno if that counts as prep or not. 

Quote from DesReploid >>

Easy. I wouldn't. Because, as the old Dungeon Master saying goes: "No plan survives contact with the players" and with mine that is often more true than not.

I do agree with this about planning. I wouldn't make plans.

What I would have is a bunch of area/region maps, city maps, NPC lists of town inhabitants, custom homebrewed magic items, monster lairs, dungeon maps, etc., ready to go so that anywhere the players want to go to, I'd have done. Instead of trying to lay the tracks right in front of the train.

I have had a similar experience... I don't have all the time in the world, but I was nominated to run our groups next campaign about a year before our current one had ended. We are in the final couple sessions now and so I am sort of looking back at what I have done and wondering if I missed anything. 

I am curious, how much info about the world did you give your players before session 1? How much did you work with them in creating their characters?

It is up to you. 

I was going to give my players a LOT about cosmology and history and then decided to age the Empire 1,000 extra years, so that this stuff is all forgotten/ancient history/unknown and part of the campaign is them learning about it, which I think is more fun.

I gave everyone some one-sheets... one on races, one on the Empire's culture and laws, one on a brief history that everyday Roman Citizens would know.

Then I set up a World Anvil site where what I've done is added more lore info to the site as they encounter. For instance, each time they encounter worshippers or temples of a god, say Apollo, I then post the lore entry for him. This prevents the players from being overwhelmed and... I think... they like it.

I'd make a big map with different regions, detail all the NPCs in each town, dot modular one-day dungeons around the land, and plant quest hooks everywhere. Then I'd finally unleash the characters into the biggest sandbox ever made.