Question: (I clearly didn't format my prior question appropriately so I'll rephrase things). 

Do MTG Core sets contain new (never before seen) cards? Or is it just cards for the prior year's expansion set? 

Assuming that there are new cards, how does this compare with the ratio of old to new cards available in expansion sets?

Answer: 
Core sets do contain new cards, but at a lower ratio than typical expansions. This [article](http://mtgsalvation.gamepedia.com/Magic_2014/Changes#Cards_added_to_Magic_2014) indicates that about half (112/249) the cards in Magic 2014 were new, while the rest were reprints. I would guess that the ratio will be about the same for Magic 2015.

By comparison, in [Theros](http://mtgsalvation.gamepedia.com/Theros#Reprinted_cards), out of 249 cards, there were 12 reprints, and 5 functional reprints (nearly identical functionality, but a different card name).
### Core sets have been discontinued, so they don't contain any cards.

In 2021, [core sets were discontinued](https://mtg.fandom.com/wiki/Core_set), with Dungeons and Dragons: Adventures in the Forgotten Realms being released in the publication slot that would have seen M22 released. Similarly, in 2022, no core set was scheduled for release, either; the game's developers have stated that their Jumpstart product was sufficient to meet the needs of introducing new players to the game.

As such we can say that, moving forward, the Core Set will contain both no reprints and no new cards, since it doesn't contain any cards, since it doesn't exist.