But what is Raiden? Was he once an Elder God? Maybe it was the old timeline but I remember it being something like he was an elder, but then gave up his status. And what is Fujin if Raiden is Earthrealm's protector? Do the Elder Gods have a limited number? Do they get replaced? What happens if they all give up their status?
We need that lore encyclopedia so bad
Look, as Misucra said, there is no written lore about this. However, the source is very clear about this.
The Elder Gods administrate the realms. They are the ones that defeated and shattered the One Being into the realms. Judging by the number of realms, the number of Kamidogu and the number of gods we usually see, we might say there were 7 Elder Gods against the One Being (6 Kamidogu + Amulet [Shinnok]), then the latter was banished. Judging by MK4/MK:A, a 7th regular God could be promoted to Elder Gods
status, so they can be replaced, at the least I think Raiden and Argus assume Shinnok's position.
Raiden and Fujin are referred as Gods. They were never referred as Elder Gods, and they obviously call those supreme beings as something they are, but none other, like themselves. There is no indication that Gods cannot die, on the contrary, many of them die, but MK
reversed dialogues that mention Gods never die, they only get reunited somewhere else (like Raiden did after being corrupted by Onaga after his sacrifice). Gods like Raiden and Argus can be
promoted to the status of Elder Gods, as it happened in MK4/MKG and MK:A, respectively. They were not born as Elder Gods but they can assume roles as if they where.
MKX showed us, on the other hand that
Elder Gods cannot die. They cannot be harmed and won't be reunited because they simply cannot die. If you cut Raiden's head, he'll die, but he'll resurrect. If you cut Shinnok's head...you know what happens. In the same game, Raiden's death is something that can happen, even if he's resurrected. Shinnok mentions about not wanting to killi Raiden and Kung Jin says the Thunder God would die. But the game is very explicit that NOTHING can kill an Elder God.
No other gods except Raiden and Argus have been mentioned as protectors of any realm. Shao Kahn is not a god, but he was mentioned by Onaga as something close to Raiden: an advisor. Raiden and Argus were very clear about being protectors of Earhtrealm and Edenia, once again respectively.
About giving up their status, it's probably chaos. They are the only entities that prevent power hungry individuals usually driven by the One Being to merge the realms. Still, they have rules, and they are a-holes. All of them would never give up their status. Raiden only did that because he wasn't one of them, and wasn't used of seeing BS and stand still. If Cetrion gives up her title, she gives up only the chair, she will forever be immortal just like Shinnok, because
she was born an Elder God.
The "Elder" name gives the notion it's superior. I see two ways: either Kronika is something superior than Elder Gods - and why not mother of them all, maybe they are brothers, maybe that what I said to Vogel wasn't very far from the truth - or she is one of them, controls an essential aspect of the universe - time, like death, life, we don't know what more -and gives up same way Raiden did in MK
A.