The rules say to pick from the suggested domains. It's mandated that the player choose from the available suggested domains.
No. That's not what the rules say. That's what you interpret the rules as requiring, and it is your interpretation that I have said is bizarre. Voadam has stated the rules:
5e PH Page 58 under Divine Domain: "Choose one domain related to your deity: Knowledge, Life, Light, Nature, Tempest, Trickery, or War. Each domain is detailed at the end of the class description, and each one provides examples of gods associated with it. Your choice grants you domain spells and other features when you choose it at 1st level."
<snip>
5e PH Page 59 under Divine Domains: "In a pantheon, every deity has influence over different aspects of mortal life and civilization, called a deity's domain. All the domains over which a deity has influence are called the deity's portfolio. For example, the portfolio of the Greek god Apollo includes the domains of Knowledge, Life, and Light. As a cleric, you choose one aspect of your deity's portfolio to emphasize, and you are granted powers related to that domain."
The same text is found
here.
And
Appendix B gives us:
Your DM determines which gods, if any, are worshiped in his or her campaign. From among the gods available, you can choose a single deity for your character to serve, worship, or pay lip service to. Or you can pick a few that your character prays to most often. Or just make a mental note of the gods who are revered in your DM’s campaign so you can invoke their names when appropriate. If you’re playing a cleric or a character with the Acolyte background, decide which god your deity serves or served, and consider the deity’s suggested domains when selecting your character’s domain.
This can be read (and I would read it) as saying that the GM establishes the setting, including the gods, and then the player considers the suggested domains - suggested by the GM, given that they're choosing the gods - and having done so, chooses a domain.
There is a degree of ambiguity over where the balance of authority lies between player and GM. That's not entirely surprising - it is typical of a lot of RPG rules-writing where PC features depend on setting details.
But it doesn't say anywhere that lists of gods and their associated domains
published by WotC have any sort of force beyond being prompts and suggestions.
The DM is the only one who has the power to add a domain to a god(suggested/examples), so if the player feels that a god who is not one of the examples should have a domain that isn't on the suggested list, he can make his case to the DM. He cannot just choose that domain for that god without that DM approval.
If you resolve the ambiguity I noted in favour of the GM, then the player can't make
any domain-related decision without talking to their GM. The lists don't have any special relevance: they're just examples/suggestions/things to consider.
@Maxperson and me think alike on that one.
Yes, I know that.
We never said a player could not make an appeal to a DM's logic and fairness. We just say that a player can not impose his non conventional choice of domain and god to the DM.
And my point is that it is tolerably clear that the whole issue of gods and domains is setting-specific. So if you regard the GM as in charge of all setting stuff, then the player needs the GM's permission to make
any choice of god and domain, whether or not it is "non-conventional".
The Appendix B lists, and the lists of names in the domains, are clearly just suggestions for possible setting choices. To quote the Life Domain text again (I think
@Voadam already quoted this upthread):
Almost any non-evil deity can claim influence over this domain, particularly agricultural deities (such as Chauntea, Arawai, and Demeter), sun gods (such as Lathander, Pelor, and Re-Horakhty), gods of healing or endurance (such as Ilmater, Mishakal, Apollo, and Diancecht), and gods of home and community (such as Hestia, Hathor, and Boldrei).
That is not a statement of a rule telling a player what gods and domains are permitted! It's a bit of advice about what makes for a coherent setting.