From the side of the deity, they are enlightened whether they manifest as a Yidam, Buddha, Bodhisattva, Lama, Protector. Therefore they all have the potential to be a yidam without a doubt. What Buddha cannot present the path according to needs the practices to enlightenment?
Take Manjushri for example:
Manifestations
Yidam-Yamantaka
Bodhisattva- Manjushri
Lama-Tulku Drakpa Gyeltsen
Protector-Four Faced Mahakala
Manjushri also manifests as horses (Ruru), Garudas, prostitutes, traders, thiefs, leaders, emperors, etc in this world, other words and other universes without count.
Although Manjushri may manifest as Yidam, Bodhisattva, Lama or Protector, from the various sides of Manjushri, they all can bring you to full enlightenment. But from the presentation of their individual paths (sadhanas-practice texts), they have different functions.
The generation/completion stages, or the four initiations in Yamantaka are not included in Manjushri's practice in form of a Bodhisattva or Four Faced Mahakala.
The clear light and illusory body meditations essential for reaching complete liberation also not found in Manjushri's practice, Four Faced Mahakala. Manjushri's practice is to generate and introduce one to loving compassion, Great compassion, bodhicitta without stressing on emptiness.
So Dorje Shugden IS QUALIFIED TO BE A YIDAM, but at this time his path is not presented as a yidam. So if you use his path to become a Buddha, by the nature of the path presented, you will not gain enlightenment which has nothing to do with Shugden's potential as a yidam himself. Similarily if you practice Black Manjushri, White Manjushri, Four Faced Mahakala, these practices although blessed will not bring you to the path of full enlightenment in itself.
TK