So a Hollow, as we know, is created when this attachment is physically broken by death but mentally the Soul is still clinging to it without letting go. This is explained as feeling grief, anger, need for revenge, and so on. But all these feelings comes from the same place - attachment. This attachment generates a sense of loss after death. And boom - we have a Hollow - with one hollow hole. A hole symbolizes the lack of heart, lost humanity, and this immense sense of loss and emptiness after being unable to move on, to let go.

I think in Espada's case their personal emptiness and this negative space, the thing they feel they are missing, and that prevents them to be whole is deeply connected with their aspects of death.

So take Aaroniero, for example - his aspect is Greed. He is missing the opposite of greed. Fulfillment. Satisfaction.

Same goes for Aizen in his Monster form imo. The first hole, the one on his chest, it the one symbolizing his heart (kokoro) which is understood as the connections with the others. The Hogyoku is filling that gap for him (visually). The Hogyoku is his equivalent of being understood. He even phrases it that way: "It looks like the Hogyoku finally began to understood who I am." - this is not what the artifact should be doing for him, this is an artificial equivalent of a human connection.

But then what about the two other holes below?

Perhaps, if the one is the connection with the others, the other one might be a connection with himself? Deeper self-understanding.

And the last hole would then represent the connection to the Three Realms, to the world itself. Because in this form Aizen is disconnected, he is on a totally different plane of existence, and the others can't even sense his reiatsu anymore. He removed himself from this existential plane.

And since they are the holes, they point out of what he misses. They suggest the absence that is not neutral, but a cause of his existential suffering (which might be not even conceptualized by him). Just like Ulquiorra who subconsciously feels there is something missing but can't grasp the concept of the Heart.

The other possible meanings of the three holes is that they could represent the Three Realms: World of the Living, Soul Society, and Hueco Mundo.

Aizen in his monster form crossed the boundaries between being a Soul, a Shinigami, and Hollow. He doesn't belong to either of the Three Realms, and three races anymore.

Next, let's look where these holes are placed. Their position is aligned with three spectific chakras:

The top one is the Heart Chakra (Anahata, the green one) representing feelings, emotional openness, and healing. Also love, compassion, harmony, kindness, and forgiveness. I don't have to explain why in Aizen's case this is empty.

The spiritual imbalance of this chakra manifests as: desire, possessiveness, arrogance, egoism, hypocrisy, misery.

Middle one is where the Solar plexus chakra (Manipura, the yellow one) would be. And this is an interesting one. Because this chakra connects the person with their power, lifeforms, will, assertiveness, self-love.

Does the emptiness here means that this form is a Dead Butterfly, an empty shell that didn't come from Aizen's own will, resolve, and conviction, but rather was forced upon him by the Hogyoku and came as a fear response? It suggest that Aizen in his form doesn't even fully believe he could win this fight, he is in denial, it isn't even him in a sense, it is an abomination of what he had strove to be. This is not what he wanted. And perhaps he does feel some self loathing for entering this form of twisted evolution.

The deregulation of the Manipura chakra causes: thirst, jealousy, treachery, shame, fear, disgust, delusion, foolishness and sadness.

The bottom one is correlated with the Sacral chakra (Svadhishthana, the orange one) responsible for the creation of things, and creation of self. Also pleasure, sensuality, sexuality.

The negative aspects for this chakra are: pitilessness, feeling of all-destructiveness, delusion, disdain, and suspicion.

I think the lowest hole in this case represents Aizen's detachment from both self, and the physical world. Or perhaps hints that his desire to create the new hierarchy is empty, and it is not an act of creation, but an act of destruction.

I think there can be also the other interpretations and meanings.

I am very interested in digging deeper into this topic, so if anyone has their own analysis, and want to share, please do!