When you go shopping in the supermarket or the drug store, you will occasionally see bottles marked "10% More!" or "25% Extra!", in which, as a promotion from the manufacturer, the bottle for one particular size of a product has been temporarily replaced with one containing a larger quantity of product.
Prior to Magic: 2010, sales of the Core Set had been lagging because the cards in that set were generally felt to be less powerful than those in expansion blocks. The change to the new Core Set format addressed this problem, but it doesn't appear to have completely solved it.
Drawing inspiration from the 12-card booster packs for Alliances, the 6-card small boosters recently made available in some mass-market outlets in the United States, and the all-foil boosters recently offered, here's how Core Set boosters could be made to contain 25% extra value, even while they still only had 15 cards in them:
Each booster pack could contain
- 1 Core Set rare (a card drawn from the rare sheet, and thus it would have the usual chance of being Mythic);
- 3 Core Set uncommons;
- 8 Core Set commons;
- One bonus card that has a 1/4 chance of being a rare and a 3/4 chance of being an uncommon, from another set; and
- Two bonus commons from that same set.
If one combined the three sets of the immediately preceding block expansion into one, the way it was done for the all-foil boosters, using that combined set as the source for the bonus cards, then all the cards in the pack would rotate out of Standard at the same time.
So a booster pack would usually contain 4 uncommons instead of 3, but it would have a 25% chance of having an extra rare instead. This might be considered a problem for Booster Draft, as it would increase the element of luck.
One other problem with this is that it doesn't seem to leave room for a basic land in the Core Set booster any longer. However, with Base Set boosters containing three basic lands each, not having basic lands in the expansion block sets or the Core Set would not be an issue.
This illustrates one way of equalizing the value between sets; if the Core Set is not as appealing as sets in the current expansion block, then put 25% extra value in it. The Base Set is an obvious source for these bonus cards, since it avoids the symmetry problem of choosing one of the three expansion sets in the previous block.
Other ways of increasing value would be applicable to other sets.
Thus, since the cards of an Un-Set aren't tournament-legal, instead of just increasing value by including full-art basic lands, maybe it should be accepted that players have a narrow view of value... and replace one of the commons in an Un-Set booster by a rare from a real set. That would automatically bump up the perceived value of a pack from essentially zero to 100% of the value of a normal booster pack.
Here, I think the best choice would be to release the Un-Set at the same time as the Core Set, and to have the bonus rare come from any one, at random, of the three sets of the preceding expansion block.
The value is slightly reduced by their having less time before rotating out, and (unlike that of the rares from the current Core Set) by being superseded, due to set design to ensure rotation, by the cards of the sets of the upcoming block.
The all-foil boosters offered first for the Alara block illustrate the principle; essentially, the boosters would be combined boosters for the whole block, except in this case everything except the rare would be replaced by cards from another set entirely.
While the Base Set is very similar to an old-style Core Set, and thus it would seem to have only slightly reduced value, if the only sought-after rares in it are Birds of Paradise and Day of Judgment , it is not impossible that this drastic a measure to equalize value might be appropriate even in its case.
However, one wrinkle might keep things from getting too far out of hand.
In a current block, with set sizes as they are now, there are 53 rares in the one large set, and 35 rares in each of the two small sets. 53 plus 35 plus 35 is 123.
Drop two junk rares, and you get 121 cards, the size of a sheet. So if the bonus rare to promote the Base Set is only a rare, and never a Mythic, printing the boosters is simplified, and the amount of value added is perhaps more in line with the amount of value that needs to be added.
This is just one of many possibilities. A bonus card that has a 1/3 chance of being each of rare, uncommon, or common, with the normal chance of being Mythic if rare, like one of the cards in the small 6-card boosters, is another option.