Could political turmoil possibly be a main focus in DA4?

It might be, but the codex only mentions the "heir" bit in Codex Entry: Queen Anora Mac Tir, which only appears if Alistair isn't sitting on the throne.

Alistair and Anora could produce an heir though

But Therin blood can overcome the taint. I don't want to get too much in spoilers but it can. If Anora is the same age as Cailan (born in 9:05) then I think it is really possible

If Theirin blood could overcome the taint, then Alistair shouldn't have been able to kill the Archdemon in DAO.

Alistair can have heir: Kieran. Another bastard prince... 

What is all this heir business. The king is voted in, didn't the Landsmeet show as much?

He's just perfect. Ferelden will be happy with him!

Actually the Landsmeet does not necessarily favour the bloodline over the "rule by merit" option. I don't know where this was originally stated, but I read on the old boards that originally the Landsmeet preferred Bryce Cousland to Cailan after Maric disappeared but Bryce was loyal to Cailan and so with the additional support of Eamon, Cailan became king. As we subsequently discover, the Landsmeet were probably right to query his suitability and his reign was only a success as far as it went because he had Anora as his queen.

Then at the Landsmeet we attend the vote can go either way depending on which nobles you have managed to win over to your side and whoever you support. It is only Eamon who seems to have this fixation with the Calenhad line and seems to think it impossible for it to be anyone else. If you play as a male Cousland then the Landsmeet are perfectly willingly to overlook Alistair's claim in favour of your own, in alliance with Anora, which all things considered would probably result in the best outcome for the country in the short term because of their ability as rulers. Then even if they didn't produce an heir there is a good chance Fergus would with a new wife, so the succession could pass to them if they seem fit to take over the role.

Apart from being a bastard, Kieran may not even exist or only as the son of the Warden, which may or may not put him in line to the throne. He is only Alistair's son if the Warden is not male or declines doing the DR with Morrigan but forces him to do so. Otherwise he is variously Cousland's bastard, slightly lower down the pecking order than Alistair's would be (particularly if Cousland does not aim for the throne); the son of Loghain (same comment as for Cousland particularly if Alistair is sole ruler); the son of an elf (no chance) or a dwarf (possible if noble).

This is why Kieran can quietly be forgotten now the writers have even removed the significance of him if he was conceived by the DR. The likelihood is that whoever took the throne at the Landsmeet will not produce an heir and so when they die the Landsmeet will decide on a new monarch from the most suitable candidates available at that time. That would certainly not include the possible bastard son of an apostate witch. Seeing as great change is being prophesised, it is likely that the succession in Ferelden would ultimately prove irrelevant anyway.

I think in the game itself though, you will see the meritocracy leaning only if you play M!Cosuland. In all other cases, Alistair's ascendancy is strongly urged by the game.

is Kieran even considered an offspring of either male or more like Bhaalspawn?

How so? I got the impression that it was only really Eamon pushing Alistair's ascendancy. True, it gives him a lot of screen time to do so, but Anora gets a chance to make her case if you choose to give her one (after it becomes clear that she's an option, anyway), and Alistair explicitly says he doesn't want it.

I don't think there's any specific precedent barring a child created in a magical sex ritual from being considered, though that's not to say that the nobles of Ferelden wouldn't happily create one. Though as Gervaise points out, the fact that Morrigan isn't married to her child's father in any World-State seems to pose a problem, judging by the dialogue about the circumstances around Alistair's conception.