Ingredient suggestions:

Filling:

The fruit: Almost any fruit will work here except for melons, they contain too much liquid and not enough structure. You generally want to keep to 3 pounds (about 9 cups) but can go as low as 2 and as high as 4, this is mostly to change the proportion of filling to topping. Dried fruit can be used as well but should be used in conjunction with fresh fruit otherwise your fillign will be too dry. I would say never go over 1 lb of dried fruit. other than that go wild with the combinations.

The cornstarch: This is used so your filling thickens and doesn't turn into a soup at the bottom of the pan. The typical ratio is 1 tbsp of cornstarch to every pound of fruit. If you are using a very watery or loose fruit it wouldn't hurt to add an extra tbsp of cornstarch. Don't go lower than the normal ratio unless you really want a watery filling.

The sugar: The sugar exists solely to sweeten your filling so how much you use will depend on your own preferences. Obviously if you use a more acidic, bitter, or sour fruit you will want more sugar. Try to stick around 3-8 tbsp total. Any sugar type except powdered will work here. Brown sugar will give more of a caramel taste and mask some of the fruit.

The salt: stick with a heavy pinch.

The spices: Any spices will work here but bear in mind some are much more powerful than others and you will need less of them. I wouldn't go over 3 tsp total.

The zest: Any zest works here try not to use more than a large orange in volume or it will taste too artificial or like a cleaner.

The extract: Any extract you'd like will work but keep in mind some are very strong. I wouldn't use more than 2 tsp or you will overwhelm the flavor of the fruit and make the filling taste too artificial.

The syrups: Any syrups will work but if you use too much your filling will end up loose or more like a sauce than a thickened filling. roughly 3 tbsp total would be best.

Topping:

The flour: The standard is 12 oz (2 3/4 cup) flour. For chocolate 10 oz (2 1/4 cup) flour + 2 oz (1/4 cup) cocoa powder. For other flours you'd like to experiment with simply replace two ounces of all purpose flour with an alternative of your choice. Remember the other flours bake and absorb liquid differently than all purpose so you may end up with a topping that is looser or more crumbly. In addition to alternate flour you can also try turning other products into flour by using a food processor. The possibilities are numerous such as pretzels, other cookies, and snacks such as chips and crackers.

The leavening: If you are making a cobbler (with the addition of milk that is) simply use 2 tsp of baking powder so you don't make dry crumbly biscuits. If making a crumble leave out leavening or you wont get a crispy topping

The additions: Up to 6 oz of fillings or about 1 1/4½cups.  You can use anything you can imagine here except anything with a high moisture content such as fresh fruit, or jams, otherwise your batter will never bake. One rule here would be nothing of too large a chunk as it will keep the crumble from staying together while baking and make it hard to eat. An example would be not using whole walnuts, you would need to break them up into smaller pieces or use them a s a sort of flour. Otherwise have fun and try whatever you can come up with. These fillings and added items should always go in with the flour to ensure they disperse evenly and don't sink while baking.

 The spices: You can use up to 3 tsp spices. You can mix and match these however you choose, going for more spice than this will start making the flavor a little too over powering and unpleasant. Spices always go in with the dry ingredients. Make sure to keep in mind that some spices are very strong so use them sparingly, such as cardamom.

For the egg replacer: You can change what liquid you use for the 2 oz (1/4 cup) just make sure it is not cold, I've used anything from coffee to apple cider before so there is a wide range. Otherwise nothing else about this portion of the recipe can be changed. As well remember to always use a neutral tasting oil at room temperature.

The nut butters: Up to 8 oz or 1 cup of nut butters can be used in the dough and in any combination. If adding a nut butter don't go below 4 oz or the crumble will be too crumbly, and don't go above 8 oz or it gets too  dense and wont cook well. 

 The sugar: 10-12 oz (1 1/4- 1 1/2 cups) sugar total. More sugar may be needed if the ingredients you are adding are earthy or bitter. This can be a mix of regular sugar or brown sugar, the later of which not only adds flavor but will make for a softer crumble. Don't use powdered sugar as it contains cornstarch. Sugar is always considered a wet ingredient.

The extract: Use up to 1 tbsp extract. Technically you can do more like some recipes listed but you will be making for a much more artificial tasting topping. Extract is always a wet ingredient.

The syrups: You can use up to 3 tbsp syrups of any kind, any more than this and your topping wont set up properly. The more syrup you add the more gooey your crumble will be when cool. Syrups always go in with the wet ingredients.

 The salt: Just stick with a heavy pinch.

The liquid: For the 8 oz (1 cup) of liquid when doing a cobbler instead of a crumble you can use anything you can imagine just make sure it is at room temperature. If you are making a crumble I don't recommend any liquid to be used at all or else you get too soft of a crumble. If making a cobbler you can use between 6-8 oz of liquid, reducing the amount if you want something drier to got with your filling. 

The citrus zest: If used will always be a wet ingredient. I wouldn't use more than two oranges worth volume wise or the taste will be off. Think more floor cleaner than crumble. Otherwise mix and match different citruses as you see fit.