I just realized what the most controversial VGU will be
The VGU where absolutely no matter how Riot approaches the problem and tries to capture the theme, there will be a not insiginifcant group of players disappointed. I mean, we got whiners all the darn time, but usually only a super-small minority. I mean, I personally love the Poppy and Sion designs and can't understand any hate towards them. Karma on the other hand... (I have not forgotten about that one Riot). However, Karma's lack of theme (or at least lack theme that translates well into gameplay) is a small problem when compared to this guy:

Cho'Gath, one of THE oldest champions. And unlike old karma, this fellow has a TON of people liking him (at least when compared to old Karma, I don't really see him much nowadays either. However, he's not totally inexistent...).
The problem with Karma was a total lack of theme. Cho'Gath however is worse: a horrible theme bad to expand on. He's just "the monster character" - at least that's what he was designed to be when league didn't even have 50 champs (weird times to remember I tell ya). The gentleman-aspect was only later added in with a joke skin...
I mean, just LOOK at him: even his design looks like a cheap clunky knock-off, like an american Zergling (meaning a very arrogant stance when talking about everything that is not his homeplace and being in a position of high power despite having questionable leadership skills and qualifications. Also, his main gameplay theme is getting fat by using questionable food sources.)
Now on a more serious note: Cho'Gath does not fit with the current Void designs, not thematically or visually. Every Void-born hungers for something, matter
information
perfection
... and then there is this big guy who is supposed to be the big boss of the pack, and he's just... hungry? For, like everything material? That's Kog'Maw's thing ya know? (on a side note, the concept of an energy-absorbing voidborn might be interesting too. As a support maybe?) The widest you can stretch it is a "hunger for power", but even that is too general. And if there's a big bad hunger behind everything, then that's his boss, not Cho'Gath himself.
And on a gameplay side, we all know how clunky the old guy is and that he neither plays like a big fearsome beast, nor like anything Void-related.
The only core aspects worth preserving in this design are "big & smart monster with a dash of purple", but that alone does not make for an intruging and interesting design. If anything, it is WAY too vague, especially when compared to Sion and Poppy, who also had most of their stuff changed, but their core identity had enough meat on it to make it work. Cho'Gath's doesn't. There is too little form a thematic side, an too much generic and totally replaceable stuff from the gameplay side.
No matter how this VGU turns out, it either develops nothing and is a disappointment, or it changes absolutely everything and these boards will probably go up in flames (as they always do, just worse) unless the rework is a design-masterpiece. This might also be the reason to why he's not #1 on the priority list. Sure, some also need it more, but I guess no one at Riot know what to do with the guy, while at the same time the pressure is quite high. No matter what you do, unless it is that one perfect solution, everything you can do is wrong. I can totally understand trying to avoid that problem, but sooner or later we're gonna run out of other champs to rework and we will have to deal with him...
&
Their lores will likely receive extreme changes(possibly retconning their place of origin) and there'll be likely some controversies about Kalye or Morgana coming off as too good or evil regardless of how they'll be written.

. They expanded from his bad them from alpha league and he turned out somewhat ok