A rather interesting exercise: what if, instead of Bolas (and possibly Ugin depending on how you see their sparks), another elder got a spark? A little AU exercise.

These are my takes:

Chromium Rhuell

Since he was the de facto focal elder before Bolas we'll start with him. According to the Tarkir flashback shamanic thing, he started off as a rather haughty, self-absorbed person but already by the end of the Elder Dragon War he began to become the person he was in the comics: with high moral standards but humble enough to mingle among humans and befriend them on occasion. Ultimately he died in a latino standoff, with his body used as a boat.

Chromium's life had many twists and turns but can be roughly divided into three beats: the arrogant early years, the humble middle ones and the "in service of bad planeswalkers" final ones. So when his spark ignites can have vastly different timeline consequences. A young Chromium might not have the same character development and just end up as a narcissistic weirdo or Bolas 0.2, though he could end up having the same character development independently and with the whole multiverse as a stage. A Chromium ascending after this character development would probably mostly play a role similar to Ugin, except more proactive and actively trying to relate to people so perhaps not as cold. A Chromium ascending at the very end could just end up in a cycle of being duped by other planeswalkers, or snap and become an antagonist.

The sheer amount of variables makes planeswalker!Chromium the most interesting subject of discussion.

Vaevictis Asmadi

Vae is consistently depicted as a bully and a coward to boot, beating up people weaker than him but relying on muscle support and defecting to the strongest party unless the stakes are high enough. As a planeswalker Vaevae would just be some sort of draconic Tibalt, except much stupider and exceptionally likely to get himself killed relatively quickly.

If he somehow manages to survive until the Mending, he'd enter an existential meltdown as he realises his mortality.

Palladia-Mors

Palladia-Mors is also a petty bully but actually physically strong and secure, with social darwinistic tendencies. Should she get the spark she'd be a pretty formidable and honestly terrifying planeswalker, if Bolas already hunted entire species into extinction for sport she'd probably do that and somehow not leave a single microbe behind. Might draw enough oldwalkers against her.

When the Mending happens Palladia-Mors would be the elder to take it best. She'd just shrug it off and keep on laying waste until her heart stops beating.

Arcades Sabboth

Arcades' life was divided into two main sections: before and after fighting Bolas. Before he was wise and a genuinely benevolent ruler, but afterwards Bolas drove him to paranoia and turned him into a tyrant. If Bolas never got the spark the latter would probably never happen at least within those circumstances, but as Arcades spreads his civilsation across the multiverse it'd be pretty easy to see him go off the deep end.

I think he'd be the closest to a Bolas analogue storywise. He certainly would try to fix the rifts... and he likely would have countermeasures against the Mending's effects. He was a strategist, so he could have had plans going back thousands of years prior. Likely would be a central antagonist, if deluded "for the greater good".

Piru

Who knows?

Arcades PW would be pretty similar to canon Azor. In fact, it might be interesting to see their agreements and differences.

For Chromium Rhuell, I float this: at the Summit of the Null Moon, both Rhuell and Ravidel were killed, but Ravidel was resurrected and his spark ignited by Kristina of the Woods. We all know what became of Ravidel after, but what if their positions had (as Ravidel wished) been reversed, and it was Rhuell who was revived as a PW then and there? I think that's possibly the most curious AU case.

Piru was Chromium mate, and was killed by Dakkon Blackblade

I meant in "who knows what would happen". I know who she is... a plot device with no character of her own.