Another sammasambuddha only will apper when there is no more true dhamma, when the path that leads to nibbana has ended.
Maitreya also will be a Vīryādhika Sammasambuddha (through more effort) Buddha.
Still this is a useless discossion. As Ajahn Chah states "If you wanna see Metteya don't practice the dhamma". Muhammad is a prophet from the Islam, it's another thing, another tradition.The future Buddha, the fifth of this kappa (Bu.xxvii.21).
According to the Cakkavatti Sīhanāda Sutta, he will be born, when human beings will live to an age of eighty thousand years, in the city of Ketumatī (present Benares), whose king will be the Cakkavattī Sankha. Sankha will live in the fairy palace where once dwelt King Mahāpanadā, but later he will give the palace away and will himself become a follower of Metteyya Buddha (D.iii.75ff).
The Anāgatavamsa (J.P.T.S.1886, pp.42, 46ff., 52; DhSA.415 gives the names of his parents) gives further particulars. Metteyya will be born in a very eminent brahmin family and his personal name will be Ajita. Metteyya is evidently the name of his gotta. For eight thousand years he will live the household life in four palaces Sirivaddha, Vaddhamāna, Siddhattha and Candaka - his chief wife being Candamukhī and his son Brahmavaddhana. Having seen the four signs while on his way to the park, he will be dissatisfied with household life and will spend one week in practicing austerities. Then he will leave home, travelling in his palace and accompanied by a fourfold army, at the head of which will be eighty-four thousand brahmins and eighty four thousand Khattiya maidens. Among his followers will be Isidatta and Pūrana, two brothers, Jātimitta, Vijaya, Suddhika and Suddhanā, Sangha and Sanghā, Saddhara, Sudatta, Yasavatī and Visākhā, each with eighty four thousand companions. Together they will leave the household and arrive on the same day at the Bodhi tree. After the Enlightenment the Buddha will preach in Nāgavana and King Sankha will, later, ordain himself under him. Metteyya's father will be Subrahmā, chaplain to King Sankha, and his mother Brahmavatī. His chief disciples will be Asoka and Brahmadeva among monks, and Padumā and Sumanā among nuns. Sīha will be his personal attendant and his chief patrons Sumana, Sangha, Yasavatī and Sanghā. His Bodhi will be the Nāga tree. After the Buddha's death, his teachings will continue for one hundred and eighty thousand years.
According to the Mahāvamsa (Mhv.xxxii.81f.; see Mil.159), Kākavannatissa and Vihāramahādevī, father and mother of Dutthagāmani, will be Metteyya's parents, Dutthagāmani himself will be his chief disciple and Saddhātissa his second disciple, while Prince Sāli will be his son.
At the present time the future Buddha is living in the Tusita deva-world (Mhv.xxxii.73). There is a tradition that Nātha is the name of the future Buddha in the deva world.
The worship of the Bodhisatta Metteyya seems to have been popular in ancient Ceylon, and Dhātusena adorned an image of him with all the equipment of a king and ordained a guard for it within the radius of seven yojanas (Cv.xxxviii.68).
Dappula I. made a statue in honour of the future Buddha fifteen cubits high (Cv.xlv.62). It is believed that Metteyya spends his time in the deva-world, preaching the Dhamma to the assembled gods, and, in emulation of his example, King Kassapa V. used to recite the Abhidhamma in the assemblies of the monks (Cv.lii.47). Parakkamabāhu I. had three statues built in honour of Metteyya (Cv.lxxix.75), while Kittisirirājasīha erected one in the Rajata-vihāra and another in the cave above it (Cv.c.248,259). It is the wish of all Buddhists that they meet Metteyya Buddha, listen to his preaching and attain to Nibbāna under him. See, e.g., J.vi.594; MT. 687; DhSA.430.