Simple)
Season of Sages - everyone gets Sage set bonus of double Death Breaths. And since everyone seems to use Convention of Elements, throw that in as a freebie, also. Opens up a ring or cube slot for different combos.
Nothing game breaking / changing or requiring massive code rewrites.
No, just no, bad idea
No, just no…
Besides the fact it’s a terrible book? Much like a few other things I’ve read lately.
Holy moly, 31 edits on OP. I think that’s the most I’ve seen. With the overall reaction so far, a favorable edit may be to select all and enter “Redacted…” 
It’s wrong to be French
Why no ?
Isn’t this proposal more interesting than Season 21, Season of Meteo ? 
It’s bad because it’s overcomplicated and adds nothing meaningful or creative.
Season of Legacy and Double Royal Grandeur
Legacy of Nightmares Effect
Ring of Royal Grandeur Effect, but with 2 items
Legacy of Nightmare can be used with 2 parts of sets
Blackthorn Set doubles effect of Legacy of Nightmare
Season 17, already past, sorry
https://us.diablo3.com/en/blog/22976066/season-17-the-season-of-nightmares-is-now-live-5-10-2019
Idea already submitted
Season 21 - Idea - Grandeur Impériale - Reduces the number of items needed for set bonuses by 2
https://us.forums.blizzard.com/en/d3/t/season-21-idea-grandeur-imp%C3%A9riale-reduces-the-number-of-items-needed-for-set-bonuses-by-2/14964
What is overcomplicated ?
Change an inventory in an array of inventories ?
OMG that’s is sooooo hard to code. 
Comment a class lock so items are not locked ?
OMG that’s is sooooo hard to code.
Mixing legendary gems is creative
Mixing legendary items is creative
Mixing class-unlocked items is creative
Mixing set items is creative
combined…
Why not, that could be interesting.
But lacks a little bit of originality 
Your suggestion is not mixing, it’s adding them together. It does not create diversity, it reduces it down to zero. You’re not adding creativity, you’re killing it.
It’s called freedom.
I know, it’s frightening at the beginning.
But once you taste it, you can never go back.
–
True but I also try to reflect constructive suggestions in the main post
I can take him. 31 edits is a typical day for me. Of course, I usually get a single post right in a couple of takes
I mean the simple fact alone that no one has liked the OP’s post… Tells it all.
This just has all kinds of over complicated on it, not to mention over powered.
Game on.
You mean your mind is not able to glimpse all the possibilities, so it looks complicated to you ?
Yes, look at that overpower
–
It lacks a little bit of originality (Basically a Season of Forbidden Archives x3).
What about cubing Sets ?
https://us.forums.blizzard.com/en/d3/t/season-21-idea-cube-all-sets/12818
Still too complicated in a cube context. However, a set (I’ve suggested Krelm’s or Blackthornes) that promotes using other set items is a GOOD idea. Something like 1100% damage per set item for a max of 11,000% multiplier is very viable.
I do agree. It’s perfectible.
That’s why I came with the multi-inventory idea. It’s wayyy easier to apply multi 6 pieces bonus Sets.
I’m curious. Tell me more
Not sure what else you need. If they opt for Blackthorne’s, you would have to make it you can’t get a class set + BT together. If it’s Krelm’s set, I’d suggest something like 5% damage/DR for every set item.