Yes, there have been many prediction threads about 6e in the past. This thread isn't mean to predict when 6e will come out, but when it does, what changes do you expect to see based on what you've seen WoTC do in the past few years in regards to errata, rules changes, design directions, etc.
For me, I think Tasha's was a signal flare of sorts. And with the recent UA, I think the writing is clearly on the wall. We will see a 6e, because some of the most cherished sacred cows of D&D are going to go through big changes on how the rules are going to be written for them. Also, when I look at the history of D&D, it seems more common than not that when you reach the point where there are a lot character options and most/all of the campaign settings are out there, we see a new edition in a year or so.
Let me address the latter first. First, let's look at the actual list:
1e to 2e: Dragonlance and Planescape settings came out, and immediately 2e discussions were being made.
2e to 3e: The Player's Options books were clearly a look at revising the rules, (and of course WoTC would want their own edition rather than TSR's 2e)
3e to 4e: 4e was announced almost immediately after the Complete X books came out (complete champion was June 2007 and 2 months later 4e was announced, so they were clearly talking about 4e long before that).
4e to 5e: 4e churned out a lot of player's options and settings right out of the gate. 3 player's handbooks, 3 monster manuals, and 2 DMGs in a 2 year period. By the time 2012 came and the Player's Options books (Feywild and Elemental Chaos), pretty much everything was covered. 5e was announced shortly after (actually announced before PO Elemental came out). Yes, sales figures had a lot to do with it, but more to the overall point:
When an edition has pretty much gone through all the core archetypes, and all of the most popular settings have been created, a new edition soon follows. I'm guessing a large factor is because not as many people buy the outlier materials. Complete book of fighters is gonna sell more copies than Complete book of gnomes. Forgotten Realms campaign setting will sell more than Spelljammer. Etc. So from a business perspective, in order to increase sales, come out with a new edition.
5e sales are still really strong, and I suspect that's because of the slow release schedule so a lot of the popular material (like settings of Darksun and Dragonlance) is still yet to be addressed. That's why it's currently one of the longest running edition of D&D ever (almost 10 years since announcement) with at least another year or two. But it is starting to see the end of the tunnel re: archetypes. With books like Tasha's we're starting to see some of the more weird and unusual class/subclass/race options.
The former point is the actual design changes we're seeing in Tasha's and the Gothic legacy UA. Similar to the Player's Options books of 2e, we're seeing some significant changes to how character creation and advancement is being handled now.
That leads me to my prediction of 6e and what we'll see and expect.
Races: Racial modifiers are gone. Caps won't make an appearance. The term "race" might even go away to something like Ancestry or Legacy (I think PF does something like this). Racial choices will have a few traits based on physiological aspects, and not cultural. A race like goliath will have a powerful build trait to represent how they are stronger. Gnomes will have magic resistance. Halfling will be lucky, etc.
Ability score modifiers and other traits will be based on culture/heritage options. Also like PF2 does I think (and a lot of indie games are doing it the same way going forward). Instead of getting a +1 bonus to strength for being an orc, perhaps you get a +1 bonus to strength for being a fighter, or choosing a warfare culture, etc. Or instead of ASIs, you get feats that are related to your culture/heritage.
Alignment: We've already seen how humanoid races are no longer inherently evil. This continues. I think no intelligent species will have a default alignment any longer. That will be saved for monsters/fiends/undead. I would not be surprised to see a shift away from the 9 alignments and go back to the B/X version of general overviews of alignments. At least for PCs. Most PCs don't follow alignment anyway, but shift back and forth depending on what's going on in the game. I doubt that will happen, but I wouldn't be shocked if it did.
Classes: A lot more subclass kits, but they will be less robust than they are now, and you may be able to choose more than one. Something between a feat and a subclass as we see them in 5e. And closer to as they appeared in the playtest docs. The reason for this, is because I think it addresses the omission of classes like the warlord, shaman, and others. For example, all fighters are good at fighting martially, but a warlord kit gives you abilities that you gain at various levels to inspire allies and enforce battlefield tactics. While a battlemaster is all about maneuvers, and a champion gives you out of combat abilities, etc. If they really want to make the change, they would get rid of subclasses/kits altogether and expand and expound backgrounds to fill that role. However they do it, I strongly suspect they will have the class as a chassis with the core features, then a lot of options you can add for backgrounds or subclass kits, and those would largely be class agnostic (warlord background with a rogue class? Why not?).
Anywho, those are my predictions of a 6e. Rather than driven by sales, I think a driving factor will be how the gaming community views design today. I.e., things like race and alignment and the problematic issues therein.
Ultimately people wanted a relatively streamlined system, not a game that greatly rewards rules mastery or plays like GURPS.
