There are several aspects of the chemistry of the chocolate cake being converted to a white cake that are not covered in the existing answers.

Fat content of cocoa powder

You cannot simply replace the cocoa powder with flour, because it compared to flour, cocoa powder:

• Contains more fat (about 10-12 percent by weight, for grocery store brands; more for some premium brands up to about 25%)
• Is more acidic
• Is far, far more flavorful (okay, that part is not desired in a white cake)

You should use slightly less flour than there was cocoa, to avoid adding flour by weight equivalent to the fat in the original recipe which would throw the balance off. I would suggest a half cup plus 2 tbls of flour instead of 3/4 cup of cocoa as a starting point.

Flavor balance

With the astringency of the cocoa removed from the recipe, the flavor balance may be too sweet. You might want to try reducing the sugar by about 1/4 cup - 1/2 cup.

Acid and Leavening

The original recipe contains both baking soda (which requires an additional acid from another ingredient) and baking powder as leavening.

When you remove the cocoa, you are removing the acid that the baking soda was intending to react with.

Therefore, you need to remove the baking soda, replacing it with baking powder which brings its own acid. You might need a slightly larger amount, as part of the weight of baking powder is the acid, so there is less leaving per unit than with baking soda. I would suggest an additional 1/2 teaspoon as a starting point.

Boiling water

The boiling water is specifically intended, I infer from experience with many chocolate cake recipes, to bloom the flavor in the cocoa. When you remove the cocoa from the recipe, you remove the need to use boiling water to enhance the chocolate flavor.

Furthermore, the reason water is used rather than milk or another liquid is to interfere less with the perception of chocolate flavor. Therefore, you may choose to use another liquid like milk or additional coconut milk which will enhance the flavor and tenderness of your final cake.

Vegetable Oil

Vegetable oil is chosen in this recipe to avoid interfering with the flavor from the cocoa.

You may continue to use it, or you may find that butter or even coconut oil give a more agreeable flavor for the final product you are creating.

Method

With all of the above changes, there will no doubt be some changes to the method. Since you have only listed the ingredients, and not the actual steps, it is harder to be specific here.

However, what will be left is a fairly standard sponge cake, so should modify the steps to the standard creaming method:

1. Sift the dry ingredients together; combine the wet ingredients other than butter and sugar
2. Cream the butter and sugar(s) together
3. Add in the eggs one at a time, beating until smooth.
4. Add the dry mix alternating two or three times with the wet mix, beating each time for 30 seconds or so
5. Beat one or two minutes longer to develop structure

Flavorings

For coconut, using coconut oil in lieu of vegetable, and more coconut milk in lieu of water, and adding coconut extract should do the trick. The recipe should also be able to carry a small amount of shredded coconut if you choose, although that will change the texture.

For vanilla, try butter and milk instead of oil and water. Add vanilla extract.

For lemon, start as with vanilla, and add lemon zest to the batter (and maybe lemon extract), and possibly, should you choose, a touch of yellow food color. I would not add lemon juice as it has a profound effect on the chemistry; instead I would make a nice lemon icing or glaze that has the tartness.

Conclusion

Despite all of the above, I would not try to convert a recipe that is specifically designed for chocolate to a white cake; too many aspects of the cake will change. Instead, I suggest finding a truly outstanding white or yellow cake recipe to use.

You say you are substituting the juices for milk. You have to ask yourself, what roles does milk play in the recipe, and how well will the juices in question fill those roles?

What does milk contribute?

The milk adds:

• Liquid, to gelatinize the starch in the flour to create the structure of the cake; the water will also contribute to some gluten formation, but in most cake techniques, this is minimized
• Very minor acid component
• A small amount of fat, but compared to the oil in the above recipe this becomes negligible
• A small amount of sugar, but compared to the sugar in the above recipe, also negligible
• Some proteins and other milk solids which will interfere slightly with gluten formation, again enhancing tenderness; they also help to retain moisture so the cake stays fresh longer
• A very milk neutral flavor that comes out as a general contribution to the richness

The fruit juice

In contrast, what do fruit juices bring to the recipe? I am ignoring the heavy/light or thin/thick distinction because it means very little.

• Water--exactly the same as the milk
• Significant acid. Fruit juices tend to be much more acidic than milk. You might wish to experiment with reducing the baking powder, and adding some baking soda to use the additional acid to power the chemical leavening, while neutralizing some of the overall acidity
• Most fruit juice is essentially fat free, but as the fat contribution from milk is negligible, this is not an important distinction
• Possibly some pectin or other long polysaccarides which will contribute to the overall structure and mouth feel of the cake, but this effect is probably negligible
• Flavor. The fruit juices will have a strong and significant flavor--of course, this may be a positive, but it will influence the outcome of the cake
• Sugar. Fruit juices tend to be sweeter than milk, so the overall cake will be slightly sweeter. This is probably somewhat mitigated by the increased acidity, and again, compared to the overall sugar in the recipe, probably not a large contributing factor.
• Fruit solids (proteins, minerals and so on). Much like the milk solids, probably contribute somewhat to the more tender crumb, but hard to characterize.

The truth is, the "heaviness" or "thickness" (by which I infer you mean viscosity, or how fast the juice flows or pours--or how easily it coats a spoon, which is an easier way to see it) is not that important.

You could use pureed fruit with almost the same result--for example, canned or pureed pumpkin or mashed banana. Even pureed fruit is still mostly water.

Still, every juice or fruit puree is its own entity. I would keep a little diary of how each trial worked out, whether you like the results, and adjust the next try accordingly.

Edit:

What juices to use?

I realized I never specifically answered the question, what juices to use?

Use whatever juice (or fruit puree) you like, based whether you like the flavor and how the cake turns out. Each time you try something new, there is some risk, but you should soon find ones you like, and which perform well.