As it follows Sigmar is the only man ever to rise to divinity but is it possible that it can happen again.
Sure. Step one - die. Step two - get lots of people to worship you. Step three - appropriate the ripples of Aethyric energy resulting from said worshipping, incorporate them into your own being. Bad-a-bing, bad-a-boom, you're a god.
Well, it's harder than that. Only an
extremely special person could do it, and it would take no small amount of luck. Simple worship isn't enough - it requires specific emotional content, and finding a niche could be difficult. Existing gods already snap up most of what goes into the Aethyr. You'd need to find an angle (i.e. what you're going to be god
of - Sigmar went for unity, human pride, strength, and ingenuity, and so on. You couldn't use that again, because Sigmar's already got it. And so on if you wanted to be god of anger and rage, Khorne got there first, or if you want to be god of death, Morr's already there, etc.) and play that up. You would also need
extraordinary willpower to be able to draw all this into yourself, fend off the predations of other gods, and maintain control over all this amalgamated energy.
That's one of the tricky things about being a god. It's not like being a mortal. As a mortal, you pretty much have free will. A god, though, is literally made of emotion. By its very nature a god will be affected in certain ways because of that. A god of anger, like Khorne, will change to match that - and so Khorne is permanently enraged. Similar gods will flow together, and the boundaries between different deities are unclear. For example, if you were Taal, you would have to acknowledge that you are also simultaneously Rhya, and yet Rhya is different to you. Or take the Lady of the Lake. Some think she is an aspect of Isha, or an aspect of Torothal, or Torothal or Isha or both are aspects of the Lady, and then there are theories that Rhya is an aspect of Isha (or vice versa) (specifically,
Liber Undivided suggests that Isha, Rhya, and the Lady are all part of the same gestalt), and then others think the Lady is an aspect of Myrmidia or Myrmidia aspects the Lady (
Tome of Salvation suggests this), and so on. For a god, something as basic as
identity can be quite fluid. Are the Lady, Isha, Rhya, Torothal, and Myrmidia five different goddesses? Are they only one seen from different angles? Are they ten goddesses, labelled in bunches? They're a big, squishy, fluid mass of Aethyric energy, and picking out where one ends and another begins can be difficult. And then you add in that, for example, Torothal can't be purely part of the Lady but is also linked to Manann, who is in turn connected to Taal, and Taal is strongly linked to Ulric (as his brother; though familial relationships in this respect are but mortal metaphors that can be used to denote strong Aethyric connections between gods), and so on down that path, it grows even more complicated. Take it far enough and you might think
all the gods are the same, to the extent that, from the proper perspective, Tzeentch
is Nurgle, and so with all gods.
So, suppose you're Sigmar, this is a problem. Sigmar has some overlap with gods such as Ulric (battle, strength), Tor (much the same as Ulric) Verena (justice, law), Ranald (defiance of fate), Asuryan (kingship, ruling), some of the dwarf Ancestor gods (if they're real gods, which is in doubt; we can be more certain about the gods of humanity and elfkind, but the dwarfs are more of a mystery), and so on. How can Sigmar's mortal soul retain a coherent self-identity in the midst of these other influences?
And if it can be done who do you nthink would have the best change of achieving it?
No one alive. Not a single person alive in the Warhammer World has the qualities needed. People capable of becoming gods are extremely rare. We're talking one person every thousand years or so, and even then, most fail. Sigmar was a very remarkable person, which is, incidentally, why
Heldenhammer portrayed him so poorly.