I like the adventures I have with my groups, but I've been thinking about creating a world where your party truly has no main objective and adventuring is the main goal.

No fixed storylines, no overshadowing plots. I'm using mapping software to create the continent with a handful or so great (capital) cities, and many more smaller cities and towns. There's politics, inter-city-state wars that the adventurers can partake in, 'quests', honestly, it's much like a MMO experience in D&D.

I'll be splitting up the map in DM-only visible factions, each with their own renown stats, so that interaction and options are limited early game. Sort of gating mechanic to fast travel options and obtainable 'quests', basically. It also makes it so you can raid a village as an evil party and be shot on sight by anyone from that faction because of negative renown.
These are just some examples how I'm planning to handling the world, a lot of the ideas are more fleshed out.

But truth be told, I don't want to force my party to be thinking too linear, I'm making this world so that they can just pack their stuff in starting town, go straight through the mountain passes, to 'gated renown areas' and continue from there. And that should be fine, it's their adventure. But the continent is huge, and either I'll be spending an entire year thinking about every single mechanic, mapping out every last cave, tree, etc, and fully writing out every potential quest.

I'm wondering how others would go about designing an adventuring plane that's nearly completely open and non-linear. Any advice is welcome.

The idea is ad appealing as it is a enormous endeavour, imho.

To create such an open setting you would need to already establish several legends, mythos, heroes, different societies for each zone of the setting, put seeds here and there and then work on improvisation based on what the characters do/where they go.

I am personally a real lover of improv when DMing, so I would suggest you keep things as vague as you feel comfortable with. For example you could put down main macro-areas, attach to them a few important settlements (villages, cities, a capital maybe) and a couple of Local legends the characters might have heard or are hearing of, and A LOT of free space underneath all this. Once you have that, you can go about filling in the details as things go on, based on what the characters do and the repercussion their actions might have in other areas (the idea of the factions is a good one to include in this).

The MMO comparison is good and not-so-useful, as in any MMO you still have, for each expansion, a storyline that develops throughout the character progression, with the end game being the culmination of that one storyline and connection to the following bit of story.

Making a truly "open ended" setting I am afraid is almost impossible, but you can do the next best thing and come up with the current storyline as things move on.

There is alot of cheap adventures that you can add.

I think the players need atleast a little stearing.

Like "There is a rumour of a ogre treasure in the south" or "the mayor needs help with a place that seems haunted"

A good kickstarter is https://www.kickstarter.com/projects/adventureaweek/mini-dungeon-tome-for-5th-edition-or-pathfinder-rp/description with about 270 adventures that can be used.

I've already planned some content steering, major cities will have adventurer guilds, with many (tiered) quests available. This is the only place where a quests is the actual proper term :D. This is intended as the most cookie cutter experience, where the party could probably get all the fun they want. 

Smaller settlements will have local law enforcer equivalents (sheriffs, outpost captain, garrison commander, etc.), with missions ranging from outlaw encampments or pirate coves (fixed to location) to skirmishes or assistance with law enforcement..

Last there will be a large pool of minor requests (which may or may not result in larger adventures). This pool consists of smaller things like killed livestock, missing forester, unnatural occurences, etc. This pool of requests I want to put into a small database of sorts, with some metadata, among others location data. If the party arrives in a port town, with no forests in sight, and ask for a minor request to do, I roll a random request with location Sea AND NOT Forest. Similarly, if there is a forest, I roll from the entire list of Sea AND Forest.

I like fantasy books and story writing, so I'll be putting in plenty of lore, mysteries, mythology and history to discover. Rumors of an ancient tomb deep within the haunted forest, things like that. These side adventures can only be found by stumbling upon it, or the party talking to the right people.

I like improv too. But say the party stumbles into a cave and I just feel like having this cave be significant, would you just improvise a floor plan and potential enemies on the fly? And something like loot on a adventurer corpse as a reward?

I'm currently thinking about having like a general area list which has something like: Enemies commonly found in snowy mountains: ... Wolves, Bear, Yeti (Rare encounter)

Quote from Maadao >>

I like improv too. But say the party stumbles into a cave and I just feel like having this cave be significant, would you just improvise a floor plan and potential enemies on the fly? And something like loot on a adventurer corpse as a reward?

Quote from LeK >>

Quote from Maadao >>

I like improv too. But say the party stumbles into a cave and I just feel like having this cave be significant, would you just improvise a floor plan and potential enemies on the fly? And something like loot on a adventurer corpse as a reward?

 You have no idea how many times I did things like these over the years :p