Allah says in the Qur'an that the only thing that will not be forgiven is that partners be associated with Allah, and that Allah forgives whatever else Allah wills.
Allah also says in the Qur'an that whoever believes and works righteous deeds, including Jews, Christians, and Sabeans, will have a good afterlife.
With that in mind, how can someone exclude some people from heaven just because they are not Shi'i (or Shi'i Ithna Ashari)?
Only Allah has the authority and power to send people to heaven or hell.
Nowadays, also, people often follow religions or sects due to inherited understandings and doctrines. A Sunni who loves certain individuals (who do not have the same status among Shi'is) does so because they have heard very good things about them, or because they believe the Prophet (S) loves them, not because they are trying to fight against the Imams or support wrong.
It is a different situation for someone who actually was alive during the time of Imam 'Ali (A) and saw him and rejected him or physically fought against him. Anyway most Sunnis, and most or all Sufis, respect Imam 'Ali and other Imams, they just have a different understanding of their role.
However, following the teachings of Ahl al-Bayt (A) through the Imams can help develop one's ethics, actions, and spirituality in such a way that can, insha'allah, help with going to heaven. Also one can hope for the intercession of the Imams (A) if one has a close relationship to them and is dedicated to them, and this is also an invaluable gift.