According to Mahayana, the arhat still need to practise for
quite some time to reach full Buddhahood. As you said Theravada
does not share this view because although Pali suttas did speak
about attaining Buddhahood by practising as a bodhisattva, the
emphasize is on attaining arhatship and little is spoken about the
bodhisattva path.
According to Mahayana sutras, I will quote two - Mahaparinirvana
Sutra and Lankavatara Sutra which are both spoken by the
Buddha,
Mahaparinirvana Sutra:
"O good man! How is one arrived, having not yet arrived?
Non-arriving is Great Nirvana. Why is one arrived? Because one
eternally cuts away greed, anger and ignorance, and all evil
actions of body and mouth; because one does not receive impure
things; because one does not perform the four grave offences;
because one does not slander the vaipulya sutras; because one does
not become an icchantika, and because one does not commit the five
deadly sins. For these reasons, we say that one arrives, having not
yet arrived. Now, the srotapanna arrvies after 80,000 kalpas, the
sakrdagamin arrives after 60,000 kalpas, the anagamin after 40,000
kalpas, and the arhat after 20,000 kalpas, and the pratyekabuddha
after 10,000 kalpas. For this reason, we say that one has arrived,
while not yet having arrived.
Lankavatara Sutra, Chapter X - Discipleship: Lineage of the
Arhats
THEN MAHAMATI ASKED the Blessed One: Pray tell us how many kinds of
disciples there are?
The Blessed One replied: There are as many kinds of disciples as
there are individuals, but for convenience they may be divided into
two groups: disciples of the lineage of the Arhats, and disciples
known as Bodhisattvas. Disciples of the lineage of the Arhats may
be considered under two aspects: First, according to the number of
times they will return to this life of birth-and-death; and second,
according to their spiritual progress. Under the first aspect, they
may be subdivided into three groups: The "Stream-entered," the
"Once-returning," and the "Never-returning."
The Stream-entered are those disciples, who having freed themselves
from the attachments to the lower discriminations and who have
cleansed themselves from the twofold hindrances and who clearly
understand the meaning of the twofold egolessness, yet who still
cling to the notions of individuality and generality and to their
own egoness. They will advance along the stages to the sixth only
to succumb to the entrancing bliss of the Samadhis. They will be
reborn seven times, or five times, or three times, before they will
be able to pass the sixth stage. The Once-returning are the Arhats,
and the Never-returning are the Bodhisattvas who have reached the
seventh stage.
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The terms 'stream entered', 'once returning' and 'never returning'
are different from the 4 classes of Arhatship we usually hear in
Theravada. Stream entered means the initial stages of enlightenment
which is from the first to the fifth bhumi, or under arhatship
class, anywhere between sotapanna to anagami but before arhatship.
"Once returning are arhats" means that the arhat has still has to
return to practise until the further stages, and has not attained
the non-retrogression of the 7th and 8th bhumi. Arhats have
attained the ability to pass into Nirvana, but this nirvana is not
the same as that of Buddha and therefore they are considered 'once
returning' according to Mahayana:
But at the sixth stage all discrimination ceases as they become
engrossed in the bliss of the Samadhis wherein they cherish the
thought of Nirvana and, as Nirvana is possible at the sixth stage,
they pass into their Nirvana, but it is not the Nirvana of the
Buddhas.
~ Lankavatara Sutra
No returning means the Bodhisattva has reached certain degrees
of non-retrogression (7th and 8th bhumi) and will not 'succumb to
the entrancing bliss of the Samadhis'.
