His government would look to devolve power in an undivided island, Wickremesinghe said.
The Tamil minority has been urging for power to the north and east provinces since the LTTE's military campaign ended in 2009.
The premier said the private lands acquired for military purposes in the former conflict zones will be returned to their owners after the election.
Wickremesinghe made the remarks - being seen as his first policy statement for the polls - while addressing his United National Party (UNP) special convention which adopted a resolution to work as a broader political front to pursue good governance and democracy.
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Sirisena, who branded Rajapaksa as a corrupt leader and resisted calls to allow him back, has also agreed to give party tickets for all Rajapaksa loyalists who face corruption charges.
The decision shocked the UNP and Sirisena's civil society allies, who blamed Sirisena for betraying the cause for good governance.
Angered by his decision to bring back Rajapaksa, a Sirisena loyalist Rajitha Senaratne said a group of them were leaving him and would align with the UNP to contest the election.
Arjuna Ranatunga, the former world cup winning cricket captain and a key Sirisena ally in the January election, is also said to be contemplating a move to leave Sirisena and join the UNP front.