1. Save up your crafting/bounty materialsYou'll need lots of crafting mats if you want to create some shiny new items using Kanai's Cube. You'll also want to cube some of the new legendary powers. Do some bounties and store the mats, you'll need quite a few. Also save up some Ramaladni's Gifts so you can stick sockets on your new weapons.
2. Save these setsThe below sets, with the exception of Istvan's Paired Blades, are pretty useless at the moment, but will be seeing a substantial buff in 2.4. The Legacy of Nightmares ring set will be quite interesting, but I don't think they'll be able to balance the power across all the classes. Start saving them if you have them.
- Legacy of Nightmares ring set (Litany of the Undaunted and The Wailing Host)
- Endless Walk ring/amulet set (The Compass Rose and The Traveler's Pledge)
- Istvan's Paired Blades (Little Rogue and The Slanderer)
- Thorns of the Invoker set (Crusader only)
3. Prepare to replace these itemsYou might have beautifully rolled ancient versions of the below items. Unfortunately most of them will no longer be BIS (best in slot). Newly dropped versions of these items have substantially more powerful stats on them. Don't save unidentified versions of these - the stats will only appear on ones that drop after 2.4 goes live. If you save up lots of bounty crafting mats then you can use them to reroll these items and they'll get the new stats on them (and it might be ancient if you're lucky).
- Dread iron (+150-200% Avalanche damage)
- Blade of prophecy (+75-100% Condemn damage)
- Drakon's Lesson (Shield Bash damage bonus increased from 150-200% to 300-400%)
- Frydehr's Wrath (+150-200% Condemn damage)
- Golden Flense (+150-200% Sweep Attack damage)
- Jace's Hammer of Vigilance (guaranteed primary attribute)
- Jekangbord (+150-200% Blessed Shield damage)
- Unrelenting Phalanx (+45-60% Phalanx damage)
- Bombardier's Rucksack (+75-100% Sentry damage)
- Dead Man's Legacy (+75-100% Multishot damage)
- Hellrack (guaranteed primary attribute)
- K'mar Tenclip (+75-100% Strafe damage)
- Valla's Bequest (+75-100% Strafe damage)
- Wojahnni Assaulter (guaranteed primary attribute)
- Gyana Na Kashu (+75-100% Lashing Tail Kick damage)
- Tzo Krin's Gaze (Wave of Light bonus damage increased from 20-25% to 125-150%)
- Vengeful Wind (Number of Sweeping Wind stacks increased from 3 to 6-7)
- Inna's Reach (Bonus Mystic Ally damage increased from 20-25% to 75-100%)
- Triumvirate (Arcane Orb damage bonus increased from 75-100% to 150-200%)
4. Clean up your stashThere are an incredible amount of new items landing in 2.4 - clean out your stash and your mules. Be rutheless!
5. Level up some gemsThere will be a new Kanai's Cube recipe which consumes a legendary gem (and some other mats) in order to add some stats to an ancient item. You can determine the stat based on what other gems you include in the cube - Ruby for Str, Topaz for Int, Emerald for Dex, and Amethyst for Vit. The amount of the stat that gets added depends on the level of the legendary gem you use in the recipe. For example if you added some Emeralds and a level 40 gem, you will be able to add 200 Dex to an item. If you used a level 80 gem you will be able to add 400 dex to an item. You can only buff ancient items. If you find an item with better stats, you'll need to farm another gem and buff the upgraded item. Because of this you'll need a lot of extra gems - you might as well get a head start on leveling them now. If you need extra gems to drop, take them all out of your stash and put them on another character. Then when you complete a greater rift you'll start getting additional gems dropping again. Level these up as high as you can so you have a bunch of them ready for 2.4.
6. Farm some goldIn 2.4 you'll be able to "empower" a greater rift. In order to do this you'll need to spend some gold. The amount depends on the level of the greater rift you're doing. If you complete the greater rift on time, you'll get 4 gem upgrade chances instead of the usual 3.
7. Save up some Bovine Bardiches and Puzzle RingsEmpowered rifts will need a lot of gold. Put together a gold find set to make the most of your puzzle rings. The new action combat feature provides some pretty hefty XP bonuses. The Bovine Bardiche can be put in the cube and it will open a special cow level. Aside from netting you a heft amount of legendaries and XP pools, the density for these levels is extremely high. On the PTR I got about half a paragon level (granted only level 975) by earning a huge massacre bonus on this level.
8. Misc notes
- Monks will need to focus on Physical Resist even more for their gear, as most rift guardians and other mobs have had their elements moved to Physical. The Mara's Kaleidoscope amulet will not be quite as good in 2.4.
- String of ears looks like it is getting changed retrospectively. This doesn't happen very often, but usually when it does the range gets rerolled again. This means if you have one with 30% (range being 25%-30%) then when the patch lands and it gets changed to a power instead of a secondary roll, it might not end up as the max value in the range. It looks like this belt will be dropped to 20%-25% (the patch notes are confusing for this).