From a mechanical standpoint, all a focus has to be is something you hold in your hand while casting a spell that requires material components. All classes that can cast spells at first level get a choice for a focus in their starting equipment. It doesn't matter to me what the player wants it to be. I mean, if a druid wants to have a pet frog be their druidic focus I'll allow it. As for multiclassing into another spellcasting class, as long as whatever the player wants to use costs the same as one of the examples in the adventuring gear It's fine by me. If a cleric/druid wants to tie a sprig of mistletoe to their symbol of Tymora so they can cast spells from both lists without switching foci or just spend time and the amount the new focus costs making a new hybrid one I think that's okay too. Personally I enjoy using component pouches. It's all fun and games until Pavel pulls out the bat guano and sulfur!
Yes, a focus is something that you hold in your hand, but the Spellcasting classes do not simply say a focus. Holy, druidic, and arcane are limiters applied to the foci. And for Clerics its 1.) holy 2.) symbol, so it's not at all arbitrary. The word symbol means the item carries some sort of cultural meaning (usually, if we're talking an "established" deity). So if the Cleric is stripped of their holy symbol, a replacement may or may not be easily improvised or attained depending on your games theology. But sure gods that straddle druidic/clerical turf or arcane/clerical turf can have a double duty focus as long as the focus is in fact polyvalent and speaks to both magics (arcane/clerical, clerical/druidic, dunno if there's arcane/druidic god out there but I imagine maybe a druid who also does wild magic sorcery employing a common element, so to speak.
Again, I'm meh on the material trappings for Clerics in most instances since we're still basically talking about answered prayers. Some gods may insist on being brandished in the act of magical intercession but not all.
I lean more towards the symbol being personally significant to the caster and their relationship to their deity than it being a major cultural one, but this difference isn't really specified in the rules. This is just a question for how defined a DM wants the holy symbol to be in their world. From the PHB:
"Holy Symbol. A holy symbol is a representation of a god or pantheon. It might be an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic. A cleric or paladin can use a holy symbol as a spellcasting focus, as described in chapter 10. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield."
The word "might" tells me that holy symbols can be more than just the examples stated in that paragraph and the ones listed on the adventuring gear table.
I, as a cleric of Oghma, might have found a weathered rock in a stream that resembled an eye. I had a personal religious experience when I first picked it up and had a revelation. From that moment on I could channel my spells through it with the blessing of Oghma. It would not matter to me that other people recognize this rock as a symbol as long as I feel a connection to my god.
If you want to run your game where only official amulets crafted at the god's temple are acceptable holy symbols, go right ahead. That doesn't mean the rules constrain other DMs from deciding differently. I think ambiguity can be even more prominent in a world like Eberron, where the gods do not walk among the people and directly influence events in an objective way, so personal faith is what powers a cleric's spells rather than it being a gift bestowed from above.
I lean more towards the symbol being personally significant to the caster and their relationship to their deity than it being a major cultural one, but this difference isn't really specified in the rules. This is just a question for how defined a DM wants the holy symbol to be in their world. From the PHB:
"Holy Symbol. A holy symbol is a representation of a god or pantheon. It might be an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic. A cleric or paladin can use a holy symbol as a spellcasting focus, as described in chapter 10. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield."
The word "might" tells me that holy symbols can be more than just the examples stated in that paragraph and the ones listed on the adventuring gear table.
I, as a cleric of Oghma, might have found a weathered rock in a stream that resembled an eye. I had a personal religious experience when I first picked it up and had a revelation. From that moment on I could channel my spells through it with the blessing of Oghma. It would not matter to me that other people recognize this rock as a symbol as long as I feel a connection to my god.
If you want to run your game where only official amulets crafted at the god's temple are acceptable holy symbols, go right ahead. That doesn't mean the rules constrain other DMs from deciding differently. I think ambiguity can be even more prominent in a world like Eberron, where the gods do not walk among the people and directly influence events in an objective way, so personal faith is what powers a cleric's spells rather than it being a gift bestowed from above.
Ruby of the War Mage is a common magic item that would work in replacing a holy symbol.
I use it for my Ranger/Cleric.