Combining various elements of the same basic taste will add that basic taste coupled to multiple textures (eg if you put sweet cherries on top of a sweet cake) and/or aromas (same example applies. Most of what we colloquially call taste is actually aroma). "Textures" can be about mouthfeel, temperature, fast or slow onset/fadeout of the basic taste, and other things. This all yields what is called "depth of flavour".
One example of both balancing and enhancing sourness is tomato sauce: Sugar is often added even though tomatoes are sweet, and (balsamic) vinegar is added even though tomatoes are sour.
Overdoing these while still keeping them balanced, though, would give you a different result - you would end up with ketchup or sweet&sour sauce (depending on texture), not something meeting the common definition of a tomato sauce.
Saltiness is treacherous since on one hand, salt balances bitter flavours (which many vegetable ingredients inherently have!) strongly - on the other hand, if you are on the brink of oversalting, salt itself becomes a metallic/bitter flavour component. Some herbs (parsley and cilantro) seem to interact with saltiness in an interesting manner, probably because they are reasonably bitter.
Richness has mostly to do with fats. The avocado in question would not infrequently be found in a salad which also has an oil or oil-emulsion based dressing. Balancing it is by acidity and textural factors (eg fresh crunchy greens on top).
Bitterness in itself is tricky. It is sometimes (not always) due to alkaline compounds (which will interact with acids strongly - sometimes by balancing flavour, but sometimes also by chemically altering each other, see baking powder...). And it is disputed whether it is really a single basic taste or a cluster of similar tastes...