look at this happy family

seen from Brazil
seen from United States

seen from Costa Rica

seen from United States
seen from China

seen from Brazil

seen from United States
seen from United States
seen from United States
seen from Uzbekistan
seen from United Kingdom

seen from Russia

seen from Germany
seen from China
seen from France
seen from Russia
seen from Benin
seen from China

seen from United States
seen from China
look at this happy family
[WARNING]
I’m aware Toby Fox said Delta Rune and Undertale are not connected. Still, I believe he may change his mind in mid development and find a way to connect the two games. No, I’m not kidding. I’m actually serious. Just hear me out.
(This will sound like a wild theory with no solid basis, so consider it idle speculation.)
The Delta Rune as explained by Gerson comes from the world of Delta Rune (game), not from Undertale
Why do I defend this idea?
In Undertale, Gerson states that its origins predate written history. Which means the events of Delta Rune must have happened IN THE PAST. If it’s an alternate past in an alternate universe, that’s another matter altogether.
According to Gerson, the winged circle represents the ANGEL, the one who has seen the Surface, while the three triangles represent the monsters.
The Angel’s Heaven (whatever that name means) is the main antagonist of Delta Rune, according to Ralsei’s legend and prophecy.
Three triangles = three heroes
A winged circle = the Angel, the Knight
There’s also the fact that the Delta Rune’s original meaning was lost in time. Meaning it could have originated from the universe of Delta Rune (game) without the monsters in Undertale being aware of that.
Now, how could the Delta Rune, the kingdom’s symbol be in the distant past if all the characters in the Delta Rune game - or most of them - are also alive in Undertale?
Something happened in Delta Rune which caused them to forget a big chunk of their past and believe the symbol’s meaning was just “lost in time”. And they believed that theory simply because no historians found any evidence as to where the rune was originated. It was impossible to find it because it didn’t originate in that strange, new world.
The truth is, the events of Delta Rune (game) could have happened only a few years before Undertale. The characters could have been sent to a different world, maybe even an alternate dimension and their memories altered to the point they believed that alternate world was the place they truly came from.
The characters of Undertale would then have NO MEMORY of their true past and just assume whatever new memories they were given must be necessarily true. A war must’ve happened against humans on the Surface. Not of Lightners against Darkners, but of Humans against Monsters. They must’ve locked us down in the underground with the help of seven mages. So, we must collect seven human SOULS to shatter the Barrier.
That would fit with Sans’ allegations that he “can’t go back, no matter how hard he tried”, that there is a character named Suzy (Susie) who we never meet in Undertale, that Gaster’s followers all look like characters from Delta Rune, that strange doors lead to strange places, much like in, Delta Rune, one particular door leads you to the Darkness, where it is said Gaster disappeared into, never to return (DARK, DARKER, YET DARKER).
And that would fit with the existence of a mysterious KNIGHT, who knows the world is just a game and its characters, merely its participants. A being capable of reshaping the world around them - as demonstrated by the creation of a second Fountain of Darkness -, and possibly even REWRITING its source code.
Such character could literally disfigure the world of Delta Rune to the point it became irrecognisable. It could alter time and space. And its inhabitants would have no memory of it, leading their lives with false memories, believing they were a king, a queen, a scientist, a warrior, a sentry in Snowdin, a doomed prince...
...and a fallen child.