For all of the history of Sanatana Dharma, there've been changes in the deities being worshipped. While of course there's been shifting between Shaivas, Vaishnavas, Sauras (who used to be much more common) etc, these kinds of changes have been most evident among 'Folk' Hindu beliefs, which are far more likely to be polytheistic and/or animistic.
Included in such changes in worship are the emergence of new deities. This has happened in ancient times (the deity Pabuji worshipped by some Rajasthanis was once a warrior, who became a mythological figure, and eventually a deity to be worshipped) and more recently as well (that murti of Narendra Modi somebody mentioned, a goddess worshipped in, I think, Kolkata representing opportunity and freedom who looks peculiarly similar to the Statue of Liberty - no I couldn't dig out the source I'm afraid). Some Hindus will have a little picture of Jesus who they do puja to in the temple.
Nice topic.
While I'm certain that the main gods of Hinduism are not going anywhere, there will be more deities added to the pantheon from different religions. I don't think that historical figures will become very big deities (or a so called "manifestation" of Brahman).
Being a Shri Vaishnava, I do include all other Gods in my worldview, although I only focus on a few deities. Being traditionalist, I am not fond of Hindus placing more emphasis on Sai Baba or Jesus than the actual Hindu Gods. Now, I totally understand ancestor worship, and I do understand why some Hindus act very universal in their beliefs. Of course, they are the good guys.
Also, might the deities of some other pagan religions, such as Celtic religions, Asatru and so on, come to be worshipped by some Hindus in Europe, as being the 'local' deities? (Personal interest because I've been getting pretty interested in Neopaganism of late)
This whole area fascinates me.
Many of the deities that many Hindus worship are not even mentioned in scriptures, the "folk deities". So I see no reason why Hindus in Europe some folk deities over there. I certainly hope that the pagan religions in Europe are not eradicated if there happens to be a Hindu majority, just like how I would not want Hinduism and its deities/shrines/temples to be dismantled and removed if another religion took over...