Marandahir
Crown-Forester (he/him)
1. There is a sort of second choice for Clerics: weapon-focused (special damage-type bonus to weapon attacks) or caster-focused (special damage-type bonus to potent cantrips). While this is usually defined by subclass choice in 5e, and reinforced by other mechanics like weapon or armour profs or bonus cantrips, as of Tasha’s it’s really just a choice for the Cleric. One could imagine that subclasses like say, Grave and Death, which have overlapping divine concepts (God of Death) but very different interpretations (offensive and violent vs. funerary and healery) could be a single domain whose interpretation is more determined by a separate pillar of cleric character development.I don't think doing something else with subclasses would necessarily be impossible. Warlocks' patron/pact boon system essentially lets them choose one subclass from each of two categories. There's no reason Clerics couldn't do the same if the developers thought there was a compelling second category of subclasses.
As for multiple domains, the issue is that we presumably want domains to continue granting mechanical benefits, so having two is only going to be fair if the second comes with equivalent mechanical drawbacks (or if everyone's expected to have two).
5e Cleric Domains-as-subclass grew straight out of 4Essentials Warpriest class’ builds, while 4E’08 and 3E had Domains as a separate pillar. 3E had mostly White Mage-y Clerics, but 4E’08 had Battle Clerics vs Caster Clerics, and 2009’s Divine Power even had a truly Pacifist Cleric build.
2. The unofficial Eberron supplement, Morgrave Miscellany, from setting creator @Hellcow Keith Baker and DM’s Guild adept (and additional designer on Wayfinder’s Guide to Ebberon) Ruty Rutenberg, features a Cleric Domain to all the Sovereign Host, essentially being a Pantheonic Cleric. This isn’t quite the multi-domain Cleric, but it is a more generalist concept with roots in the game and historic lore.
One could also imagine a One D&D concept that allows internal multiclassing - letting you take a MC level in your own class to gain a second subclass? Or perhaps 4E style MC feats that give you a variation of it.