Midsummer Battlefield Land
: Add to your mana pool.
During your opponent's turn, Midsummer Battlefield loses all other abilities and becomes a Mountain.
Imperative Delta Land
: Add to your mana pool.
During your opponent's turn, Imperative Delta loses all other abilities and becomes an Island.
Just a simple concept that allows players to splash instantsfrom other colors for quick-play during the opponent's turn. Considering the aspect of mathematical proportion in deckbuilding, I would think to add an ability that increases the maximum number of these cards in a deck to 6 copies. During the opening hand, this helps to effectively push the success rate above the abysmal 48% percentile clench. For what this does (or aims to do), effectiveness would critically rely on this increased probability ratio. Otherwise, especially in your metagame, this will fall to the same obscurities as many other non-basic lands in the past (such as painlands)—that just didn't quite make it (have the functionality necessary for success)—especially due to the probability issue of access (being limited to 4 copies).
I would further suggest that this implementation take a more stylish form, such as I believe should have been done for the Temple lands, by making a sub-type that has rules attached to it. I strongly believe that Temple should have just been a sub-type, with the rules of "Whenever a player has a temple enter the battlefield, that player may scry 1." This would be the same, only having the rules, "A deck can have up to 6 copies of a [subtypeland] with the same name."
The lands themselves aren't bad, there is a bit of similarity to River of Tears with sightly different timing. Its a little weird that they aren't Plains on your turn, but become a basic land type on your opponent's turn.
Note, though, sub-types don't have inherent rules functionality. Since Kamigawa block when all Walls were given Defender and all Legends instead gained the Legendary super-type, the game doesn't recognize sub-types impacting gameplay except as a marker for other abilities to refer to. Abilities like this are spelled out on the card itself so that players don't mis-interactions because the rule are not in the same place as the card.
The rules are weird about giving lands basic types, in that it takes away all other abilities of the land... including the one that gives it the basic land type. A better way would be:
: Add . If it's not your turn, add instead.
Note that in many games you will have more than one opponent.
I agree with the above poster that keeping subtypes free from rules baggage is a good thing. There is no need to have 6 of these in a deck.
I understand the crunch it probably creates to think that a land isn't going to be functional with Fetchlands. However, let's consider that it's not really intuitive or healthy for everything to do or be that. There should be some means for control and moderation, by forcing oneself to veer away from that to preserve it's uniqueness and prevent its shine from being adulterated and worn out.
Also consider that with the quantity rule, it doesn't really need to either.
Interaction with fetch lands isn't the problem. Its just disconcerting that it sometimes has a basic land type and sometimes doesn't. Makes it feel like something's missing. Either should have a basic land type all the time or none of the time.
Tadpole's wording is cleaner and causes less rules issues, so you should stick with that.
The real issue with these is that they are not effective as dual lands. The power of a dual is in being able access multiple tools of your deck at the same time. These, instead, push you to build a deck when most of your permanents and sorceries are one color, and your instants are another, but only for the color pairs there are duals for.
Thought experiment: If you were putting these in a set, usually there are 5 rare duals or, at most, 10 common duals. Even if you only print allied pairs, you really want two of each color pair with the order swapped (a U with W during the opponent's turn and a W with U on the opponent's turn) or deck building becomes more limited. How many spots in a set do these take up?
These would be nice supplemental lands. You shouldn't run them just to splash as they limit your options. (Sometimes it's better to fatal push on your turn opposed to their turn for instance. Like against a counter control deck you don't want allow them a chance to draw a counter.) And they aren't fetchable so this decreases the chance they are in play when you need them. So not overpowered but also just good enough. Better than the lands in double masters.
Land
: Add to your mana pool.
During your opponent's turn, Midsummer Battlefield loses all other abilities and becomes a Mountain.
Imperative Delta
Land
: Add to your mana pool.
During your opponent's turn, Imperative Delta loses all other abilities and becomes an Island.
Just a simple concept that allows players to splash instants from other colors for quick-play during the opponent's turn. Considering the aspect of mathematical proportion in deckbuilding, I would think to add an ability that increases the maximum number of these cards in a deck to 6 copies. During the opening hand, this helps to effectively push the success rate above the abysmal 48% percentile clench. For what this does (or aims to do), effectiveness would critically rely on this increased probability ratio. Otherwise, especially in your metagame, this will fall to the same obscurities as many other non-basic lands in the past (such as painlands)—that just didn't quite make it (have the functionality necessary for success)—especially due to the probability issue of access (being limited to 4 copies).
I would further suggest that this implementation take a more stylish form, such as I believe should have been done for the Temple lands, by making a sub-type that has rules attached to it. I strongly believe that Temple should have just been a sub-type, with the rules of "Whenever a player has a temple enter the battlefield, that player may scry 1." This would be the same, only having the rules, "A deck can have up to 6 copies of a [subtypeland] with the same name."
Note, though, sub-types don't have inherent rules functionality. Since Kamigawa block when all Walls were given Defender and all Legends instead gained the Legendary super-type, the game doesn't recognize sub-types impacting gameplay except as a marker for other abilities to refer to. Abilities like this are spelled out on the card itself so that players don't mis-interactions because the rule are not in the same place as the card.
: Add . If it's not your turn, add instead.
Note that in many games you will have more than one opponent.
I agree with the above poster that keeping subtypes free from rules baggage is a good thing. There is no need to have 6 of these in a deck.
Also consider that with the quantity rule, it doesn't really need to either.
Tadpole's wording is cleaner and causes less rules issues, so you should stick with that.
The real issue with these is that they are not effective as dual lands. The power of a dual is in being able access multiple tools of your deck at the same time. These, instead, push you to build a deck when most of your permanents and sorceries are one color, and your instants are another, but only for the color pairs there are duals for.
Thought experiment: If you were putting these in a set, usually there are 5 rare duals or, at most, 10 common duals. Even if you only print allied pairs, you really want two of each color pair with the order swapped (a U with W during the opponent's turn and a W with U on the opponent's turn) or deck building becomes more limited. How many spots in a set do these take up?
I think that moderation is best for the focus and enjoyment of each land, and lets people soak it up comfortable.
White to Blue gets its time to be enjoyed and shine. Then White to Red. Then White to Green. Etc.