I don't think you give players enough credit when you state that 3 items will make the jungle too complex. I feel that by the time you are equipped to jungle: around lvl30 with adequate runes (it can be done earlier, but not without struggling), you should have a firm grasp of what items will be best suited for each jungler; kinda like how there are 3 doran's items, and people generally know which one fits their champion the best (though not all of the time). People understand build paths well enough to know that you should grab a longsword if you want to build brutalizer or cutlass with it later, and the same can be said about things like Amp tome and ruby crystal (though now all health items build out of it).
I don't like his suggestion on quill coat as it does make the item way more complex than it needs to be. Give it Sapping barbs and that's all tank junglers really need, not some overly complex grey health system that acts like a Sion shield to burst down camps. But besides that I like the idea of having 2 or 3 jungle item starts for junglers as this is the only way to put junglers on nearly equal grounds when starting the game which is where real problems arise as a Tank or AP jungler. Machette currently only REALLY favors AD junglers.
Honestly, new players (lvl1-30) probably shouldn't jungle as it is one of the more macro oriented roles (if not the most) in the game where you have to use more than just mechanical skill to play correctly. A new player without as much experience will have a lot of trouble in the jungle regardless of having to choose a starting item. Recommended items are also a thing for new players to utilize and it wouldn't be hard for the Gameplay team to add respective recommended items for each jungler in the role they are most prominently played: Hunter's Machette for Vi, Quill Coat for Sejuani, and Spirit Stone for Elise.
Now a more experienced jungler may look at the matchup and say to themself "you know what, I picked Vi, but I think we are going to need a tank more than extra damage for this team, so I'm going to use Quill Coat instead so I can have access to Juggernaut/Cinderhulk", the same could be said for taking Spirit Stone on Sejuani (for Magus) if you wanted to use an AP build, Quill Coat on Elise for a tankier build, or Eve with Hunter's Machette for her AD build.
A player new to the jungle might just build the recommended items and can still do rather well just as someone can build all of the recommended items for Garen in the Toplane and do well without considering the matchup. If you think that having multiple items in the jungle is overly complicated for new players, just consider how complicated this game actually is. It takes months to years to get a decent grasp of all of the moving parts that make this game work, adding a little bit more complexity to the game will not suddenly break it. A lot of items have different build paths and a lot of us have these things memorized through experience, and that's all it takes for someone to get an understanding of items and build paths and really that's all we are talking about here as these items are kinda tailor made for each style of jungler and build into the items that they will plan on using.
On the topic of it will be more difficult to balance these items as the stronger one will always win out. People are mad about Runeglaive because it pigeon-holed AP junglers to basically only be those who use Spellblade effects well. If you were to reintroduce Magus with a passive that would favor pure mages over AP fighters, you wouldn't see Fiddlesticks grabbing Runeglaive even if it was technically stronger and on the reverse side, Diana probably wouldn't want Magus as her kit synergizes so well with Runeglaive (one could argue that she could buy Lichbane as a replacement and just use Magus but that would be the players choice, and likely, she would be better off getting Runeglaive instead of delaying her power spike). If one item is the only one seen in play (let's say noone uses Runeglaive), you would want to buff the item or nerf Magus as it shouldn't be a hands down superior item on all AP junglers.
We already see this in Devourer and Warrior, almost any champion who can use Devourer can use Warrior as well, but the items are different enough, that champions that synergize better with Devourer will pick the item over Warrior as they feel it will give them more power for that particular playstyle. Juggernaut and Cinderhulk are different enough in that Juggernaut will give you more immediate HP when you buy it and with Stonewall's suggested change, it would make for a stronger initiator type tank/fighter (this is made even more important now that Righteous Glory has been nerfed), whereas Cinderhulk would be a better option for tanks who want to reach that lategame hypertank status with 500+ bonus HP from the Cinderhulk passive (with some extra AOE damage).
This is really the only set of items that I think would be more difficult to balance as the tankier junglers can easily make use out of either of these two unique roles (with some heavily preferring one over the other). Whereas in the case of the Warrior/Devourer and Runeglaive/Magus, each of these sets would have items tailored to Fighters and Carries. The distinction between a Juggernaut style run-at-you Tank and a damage soaking hyper Cinderhulk Tank isn't as clear, possibly leading to tanks/tanky fighters only picking the one that is the strongest at the time (which is sometimes dictated by the meta).
Anyway. That's my rant. You can check my rant below to see how I come up with a new Magus Enchant that would be preferred by the pure Mage junglers and write your opinions. Later homie. Peace.
). I do think that this idea is still solid, but needs tweaking to not just favor the highest base damage junglers.
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, but for many others that could be junglers if they didn't have mana problems like 
or 
Runeglaive single-handedly made better, and the only reason it didn't make them viable is because they can't use Sheen procs well outside of its mana regeneration case.
