Does Mahayana Buddhism discourage attaining enlightenment under the Gautama Buddha Sasana? As far as I've heard, Mahayana Buddhists aspire to become Buddhas themselves in the future or want to become followers of a future Buddha like Buddha Maitreya. So, are Mahayana Buddhists discouraged from attaining enlightenment within the Gautama Buddha Sasana? If yes, wouldn't that be limiting the scope of enlightenment compared to Theravada Buddhism? Wouldn't that also make the taking refuge in the Triple gem meaningless, as they do not want to follow the teachings of the Gautama Buddha to attain enlightenment.? If the answer to the first question is 'no', any links to Mahayana specific texts which encourage one to attain Nibbana within this Sasana would be appreciated.
In early Buddhist texts one gains nibbāna and is freed from ever having to be reborn again. Many people achieved this and in relatively short time frames. The Buddha of the early texts seems to have expected the enlightened practitioners he left behind (hundreds if not thousands of them according to the texts) to teach others how to achieve this. If each disciple themselves freed only two other people, the whole world would soon be free. The traditional Buddha told his followers that the Dhamma should be their refuge and teacher when he was gone. 

However by the time the Mahāyāna sutras were being composed a significant level of doubt and insecurity seems to have crept into their minds. At the same time they began to inflate the role and the capabilities of the Buddha in their retelling of the stories. (This trend is covered in my article: Escaping the Inescapable: Changes in Buddhist Karma. Journal of Buddhist Ethics Vol. 21. 2014). The dilemma is this: if the Buddha was truly compassionate, how could he die and leave us behind? Something of this dilemma is openly expressed in the opening of the Suvarṇabhāsottama Sutra (Fantastic Golden Light) in which Ratnaketu asks why the Buddha only lived 80 years if merit increases lifespan and the Buddha had infinite merit. The answer is that the Buddha only appeared to live and die, but that in fact he is eternal. This is typical of the hyperbolic response of the Mahāyāna and the gradual conversion of Buddhism into a more theistic religion, with Buddha as God. 

In order to deal with this problem the Mahāyāna intellectuals conceived of the bodhisatva as getting to the threshold of awakening, and having more or less all the advantages of being awakened, but not attaining nirvāṇa which would ensure they could not be reborn. In this view the bodhisatva eschews enlightenment in favour of repeated rebirth to come back and help beings. In this view also we see a vastly inflated role for the bodhisatva. The Mahāyāna seems to have anxiety about the Dharma not being sufficient. One needs the (increasingly god-like) intervention of a Buddha or bodhisatva. Interestingly some Theravādins also lost heart. This view is seen, for example, in Peter Masefield's book Divine Revelation in Pali Buddhism. Masefield, a respected translator and practising Theravādin, argues that without the physical presence of a Buddha nibbāna was not possible - and that the possibility of enlightenment would have died out within a generation of the Buddha's death. It's a view that is relatively easily refuted, Masefield cherry picks his Pāḷi quotes and studiously ignores any contradictory information. (see my unpublished review of his book).

Thus we are left with the image of the bodhisatva with many magical or godlike powers, a kind of messianic saviour figure, who helps otherwise helpless beings by being repeatedly reborn amongst us to lead us to the goal that we are otherwise incapable of reaching ourself. 

In many ways this is a profoundly pessimistic view of humanity, though a very positive view of bodhisatvas and buddhas. It reaches its apotheosis in the teachings of the Japanese Buddhist, Shinran, who entirely abandoned the idea that human beings could become enlightened and preached reliance on the vows of Amitābha (recorded in the Sukhavativyūha Sūtras) a Buddha who lives in another universe but who has promised to meet people after they die and lead them to his Pureland from where they can become enlightened with no effort.

So yes, in Mahāyāna Buddhism one typically vows to become a Buddha, but a Buddha once dead cannot be reborn, so one, as it were, puts off Buddhahood until all beings are free, which at the present rate of beings being freed is essentially forever.