One question though...and from my worthless opinion...
Why should one think that in one direction of the universe that there is only one world system?
E.g In a rough sense, since I live in the South East Asia region, when I talk of the Western Direction, there are so many countries in that direction, ranging from India all the way to the Americas and each of these countries have their own respective system of governance and ruler, some of which may share similar ideals of governance and leadership, some may not. But this imagery is within one world system.
On a larger scale, is planet earth the only world system in the entire universe?
So one world system or what some term it as 'lokadhatu' is not the entire universe but rather perhaps only a tenth thousandfold of that universe not to speak of in a single direction. Have not the Sutras and Sastras spoke of different worlds upon worlds? So, based on the above, this does not contradict the teaching of one Buddha per world system. And in Mahayana, because of the Trikaya teaching, each of these Buddhas can emanate in any part of the universe without any impediment. That's why you can read in Mahayana Sutras, in various Dharma Assemblies, Bodhisattvas and Buddhas of other worlds emanate themselves in our world system.
In fact, not to talk only of Akshobya Buddha, in the other 'Sūtra of the Original Vows of Seven Medicine Buddhas' (so far I have only seen it in Chinese translation and is also found in the Tibetan Buddhist Tradition), it speaks of the other 6 Buddhas (some call them the 7 Medicine Buddha 'Brothers' and when paired with Sakyamuni Buddha, as seen in some iconography, it is eight Buddhas) alongside the popular Medicine Master Lapis Lazuli Light King Tathagata who has the foremost 12 Vows whilst the rest have number of vows ranging between 4-6 in the Eastern Direction.
More on Akshobya Buddha
and an interesting comment here:
Akṣobhya appears in the "Scripture of the Buddha-land of Akṣobhya" (阿閦佛國経 āchùfó guó jīng), which dates from 147 AD and is the oldest known Pure Land text. According to the scripture, a monk wished to practice the Dharma in the eastern world of delight and made a vow to think no anger or malice towards any being until enlightenment. He duly proved "immovable" and when he succeeded, he became the buddha Akṣobhya.
Akṣobhya is sometimes merged with Acala (Japanese: 不動明王 Fudō myō-ō), whose name also means 'immovable one' in Sanskrit. However, Acala is not a Buddha, but one of the Five Wisdom Kings of the Womb Realm in Vajrayana.
Prior to the advent of Bhaisajyaguru (Yakushi), Akṣobhya was the subject of a minor cult in Japan as a healing buddha, though even now both are found within the Shingon school of Buddhism in Japan.
Recently, newly discovered Gāndhārī texts from Pakistan in the Bajaur Collection have been found to contain fragments of an early Mahāyāna sutra mentioning Akṣobhya. Preliminary dating through palaeography suggests a late first century to early second century AD provenance. More conclusive radiocarbon dating is under way. A preliminary report on these texts has been issued by Dr Ingo Strauch, with a forthcoming paper on Akṣobhya texts expected soon.