This is the IR degree in me, but I dislike how in Arcane they watered down/completely removed the geopolitical antagonism between Piltover and Noxus that was in the original lore all just to push a really really contrived "unity of oppression" theme that makes extremely little sense, is much worse foundation for future storytelling, and is more boring to boot (a true microcosm of all the issues Arcane has caused).
To those watching at home, Piltover in the pre-arcane lore always had an under-the-table hostile position toward Noxus from the get-go. This wasn't because of some specific event (because relying on specific events for state-v-state antagonism is over simplistic) but because of actual geopolitical logic. Piltover wasn't hit with the stupid bat like they were in Arcane. Noxus is a rapacious, murderous empire that is extremely open and explicit about their desires to conquer everything (most empires are usually not so obvious). Noxus has already conquered much of the territory surrounding Piltover, and Piltover itself is an incredibly important strategic location that is the only link between the Eastern and Western oceans. Even ignoring Hextech and general wealth they've accrued, Piltover would always know that they are very close to being on the chopping block.
So what does Piltover do? It actually had half a brain unlike in Arcane and had sought to bilk Noxus for everything it can and placate them where possible while distracting Noxus with irrelevant issues using economic influence, espionage, and misdirection. It knew that it cannot fight an existentialist total war with them (although Pilt's navy was considered a fearsome one), so it sought to keep itself out of Noxus' gaze while getting it entangled in irrelevant theaters of war and internal rebellions/court intrigue that ensure that it can never bring a strong, collective response against Piltover.
This takes many forms and some are more morally dubious than others. The most morally dubious one is that Piltover does facilitate some Noxian expansionism. Sea passage through the Sun Gates has allowed Noxus to expand westward at a significantly faster pace than before (Swain's conquests in Shurima would not have been possible), although Piltover does charge exorbitant fees for their military passages to the point of bankrupting the previous Noxian regime.
At the same time on the more incidentally morally upstanding end Piltover actively provides and sells arms and material to anti-Noxian resistance movements in Shurima (like the Suns of Bel'Zhun) and to independent powers like Stonewall and various Ionian factions resisting Noxian incursions.
Piltover had interesting things going on in the wider world and the institutions that were a wealth of storytelling potential that add good conflicts and depth to the region. Things like the Inteligencers of the major Piltovan Clans like Camille offered interesting subterfuge and and espionage activities in games of tit-for-tat against Noxian Warmasons and Black Rose incursions. But we can't have those because for Arcane they needed Piltover to be bunch of dumb-as-rocks morons. All to make Noxus look "cooler".
It's also not even an unheard of situation that a smaller power can deter a significantly larger one by being a porcupine. People often fall into imperialist propaganda and assume that an empire that desires to destroy something can *always* do so. Athens was a hornets nest that the Persians for centuries preferred to never outright provoke it because the cost of doing so was so prohibitive. Theoretically Athens could be crushed by Persian size, but locally Athens had too heavy a naval presence in the Eastern Mediterranean for them to want to bother and so they had to find proxies and indirect means to weaken them (this ended up being Sparta). Where Athens and Piltover differ is that the former was desiring to be an imperial power while the latter is not so more diffuse methods of deterrence are politically palatable and available to Piltover.
There's much more avenues for storytelling by leaning into conflict between Piltover and Noxus than in making them buddy-buddies.
Ironically, pre-Arcane it had been Zaunites who have provided the most explicit material support for Noxian imperialism in the form of chemtech expertise, weapons sales, and direct assistance in several genocides during the Ionian invasion (don't worry though, Rito always ensures that there's a "good" Noxian to make a sad face at it though so we know they don't approve...). This was an interesting situation as we can understand and even empathize with the reasons why they'd do this. Gotta get that cheddar as one might say when the avenues for opportunity in Zaun itself are so limited. But nuance like the sympathetic Zaunites doing bad things to people because they benefit from the suffering of people that don't affect them is apparently too much for the writers (or the fandom, probably) to handle.
Sure, you could argue that Piltover could "develop" some of these things post-Arcane but that's fundamentally lamer angle. Tension and conflict are best in this kind of storytelling when two groups are at rough equity to each other. So many famous Cold War stories and spy thrillers don't work if the CIA and KGB aren't roughly equivalent or at the very least have strong sets of unique skillsets. It's the difference between setting yourself up from the get-go for a story and being forced to wait for the narrative equivalent of Paid DLC in the form of narrative progression to get the kind of content that we could have had from the start. Not everything within a setting needs a dedicated origin story. Not everything needs to be an underdog story.