Here are some things you can do to prepare, as well as some lesser-known things to work on. I will update this guide when we get more info when Iceborne launches on consoles. Also note there will be minor SPOILERS here.

Update: Since Iceborne has now been released, its better to simply play the expansion instead of worrying about everything on this list before you start. The only real requirement is that you finish the story, and have a decent build to get you started (defender gear will suffice).

There has been a lot of discussion of the more obvious things, and most people are probably well aware of this stuff, but I'll put it here for the sake of completion. Look further in the guide for some more obscure stuff.
These will definitely help save some time in early MR (Master Rank, the new quest level in Iceborne):

Most people will be well aware of this, but it'll be good to have lots of consumables ready for MR (I like to keep everything above 100), such as:

• Mega potions
  
• Max potions
  
• Lifepowder
  
• Dash juice (use elder melder to get lots of these, discounted during events)
  
• Demon drugs + powder (everyone should use these)
  
• Armorskin + hardshell powder (best value for gunners or support builds)
  
• Might/adamant pills (lower value than other buffs)
  
• Traps
  
• Bombs
  
• Traq bombs
  
• Flash bombs (getting nerfed in Iceborne - not worth hoarding too many)
  
• Dung pods (will be farmable in Iceborne)
  
• Farcasters
  
• Null berries
  
• Herbal medicine
  
• Astera jerky
  
• BOOMERANGS (don't actually)
  
• Loads of ammo/coatings

And the following raw materials (these can be good to bring on a hunt and craft things on the go):

• Honey
  
• Bitter bugs
  
• God bugs
  
• Blue mushrooms
  
• Nutrients + mega nutrients
  
• Immuniser
  
• Nourishing extract
  
• Catalyst
  
• Might/adamant seeds
  
• Mandragora
  
• Devil's blight
  
• Large barrels
  
• Ammo/coating crafting materials

If you have lots of spare zenny (which most people will), spend it on loads of potions, trap tools, large barrels and ammo. These will also be discounted during events.

Having a good collection of different weapons ready to be upgraded into MR will be a good idea. You will likely be able to craft MR weapons, but this will be more expensive than upgrading. We don't know which weapons will be meta (I'll add optimal trees to this page once console launches), but having access to most trees for your favorite weapon types wouldn't hurt.
Based on some early details:

• Having the ore/bone trees maxed will give you the quickest and easiest upgrade into MR weapons.
  
• MR Great Jagras weapons are also surprisingly decent for early MR.
  
• MR Pukei Pukei weapons are reliable given how poison scales in early MR and easy to craft.
  
• Acidic Glavenus weapons seem to be the best for raw damage, and for most weapons they branch from Diablos.
  
• Gold Rathian weapons are solid endgame weapons, so have HR Rathian weapons ready.
  
• Ruiner Nerg weapons are also great endgame weapons, so have HR Nerg weapons ready.
  
• For a complete list on weapons to craft, go here

While you can get by if someone in your lobby has all the ingredients, it is worth spending time collecting all ingredients yourself. People really underestimate food skills, and having more ingredients increases the chance of having fresh ingredients. There will also be new ingredients added in Iceborne too.

Charms will have upgrades in MR so having a collection of the best HR charms ready will save time, especially charms like handicraft. A full guide to the best charms can be found here.

• Decorations: HR decorations will be less valuable, as it will not only be easier to obtain rarer HR ones with new levels of feystones, but there will be new MR decorations that give multiple levels and combine skills. Still good to have a solid collection (at least of the basics like health boost), but probably not worth farming for hours until you get that elusive mighty bow jewel.
  Update: MR gear still has a lot of 1-3 level decoration slots, so having a good collection will allow you to build more versatile sets (or an excess for melding), which can be done easily by farming the event quests 'Greatest Jagras' or 'The Names Lavasoith'. Namely, make sure you have weakness exploit (still good despite nerfs in Iceborne), crit boost and crit eye (see a complete list here[docs.google.com]). However, getting rarer decorations will be easier, and there are also alternate ways of getting rarer skills (such as attack) from new MR decorations that combine skills or given multiple levels of the same skill.
  
• Kulve Taroth weapons: despite being the some of the best weapons in the game, these will eventually become redundant compared to MR gear, especially since they cannot be upgraded. Though they are still strong enough to carry you through early-mid MR. There are rumors that the devs want to reward people who have a good collection of kulve weapons, so it could be useful continuing to farm these.
  Update: This is based on a throwaway comment from a developer, and we have seen no such mechanic so far, but this may change if KT is introduced to MR. I wouldn't count on it though.
  
• Armor: again, armor cannot be upgraded, so will become (mostly) redundant in MR. Only craft what you need for early MR quests. Having meta sets such as the Drachen armor may still be useful in terms of skills, but at the cost of defence.
  
• Armor spheres: there will also be new tiers for armor spheres, so having a large hoard of HR spheres will only really be useful for early MR.
  
• Gold wyvern prints: there will be a new tier of tickets called celestial wyvern prints. Gold prints will still be useful for crafting HR weapons (or charms) that can be upgraded for MR however.
  
• Augments: apparently, augments will be kept even on MR upgrades for weapons, so having a good collection of streamstones might be useful. This is quite speculative as there will probably be a new form of augments or another means to obtain them.
  Update: Augments are kept in MR until you upgrade that weapon, then augments are refunded. Once the weapon reaches its max upgrade again for MR it you can augment it again, but only using MR materials.
  Update 2: In Iceborne streamstones can be melded into warped feystones with high commendations.
  
• Vouchers: farm greeting the gluttons for these, though there will be a new tier called gourmet vouchers.
  
• Layered armor: for those completionists, go ahead and do this now, but it will be far easier to defeat AT monsters and extreme behemoth with MR gear. Feel free to wait if you aren't up to the challenge (though these events won't always be available).
  
• Research levels: having all monsters fully researched may save time when Iceborne drops, as there will be new research levels for MR (and an achievement for maxing all of them in MR). This might be drastically faster once Iceborne drops, so this one is up to you.
  
• Event quests: prioritize event quests according to this post.
  
• Mantels, Palico Gear, Canteen etc.: These will receive upgrades in MR so you should complete your collection.

Here are some of the more obscure things which might be helpful to have done once the expansion drops or those looking to squeeze every drop of content out of the game during the wait. If you're really desperate for more stuff to do, this is for you:

While you've probably unlocked all the palico gadgets by befriending tailriders, you might not have maxed unity for all maps. This will be more important for Iceborne since it will improve the raider ride ability (and the max unity will be increased from 5 to 10). Gajalaka's are also especially important for the traps that randomly spawn on maps.

This one is a little more speculative, but event gear such as the Wyvern Ignition will likely have a MR upgrade. It is possible that these upgrades will require tickets from the low/high rank event quests, and it would be a pain if you had to wait for a given event to be available again, so might be good to have a few spares. These might also be used for more layered armour, but that's just speculation.
Update: We have not yet seen upgrades for event weapons, so this probably isn't worth your time.

This one is a bit niche, but the new housing decorations are confirmed to be bought with resource points. If you want to obsessively collect this stuff (like me), having a lot of excess RP will let you unlock everything quickly. RP is also used for layered armour, augmenting, melding, etc. so can be useful to hoard regardless. You can find a good guide to farming RP here.

Again for those completionists, having all the endemic life and fish before Iceborne will allow you to flex on people who lack the will to reload an area for hours until a giant platinum fish spawns. They will be allowing you to invite other people into your room too, so all the more reason to waste time doing this stuff.

Hopefully this keeps you busy through the long wait until launch. If you're done with world, I'd recommend checking out some other monster hunter games (I'd personally recommend 4U and GU which have plenty of content) or even Dauntless for a few hours until you get bored of it and go back to MH (sorry Dauntless).
I'll try to keep this updated as we get more info, and let me know if I missed anything.