Quote from Firebead_elvenhair »About planeswalkers working together, there are precedents, like the Nine Titans or the closing of the Time Rift (which Bolas obviously knew). Not forgetting Sorin, Nahiri and Ugin. So it could happen, but it is a very rare event... Not worthy of building a plane around it, in my opinion.
I think that the cunning of Bolas has been vastly overrated (from fans and creative alike): it's difficult to give him a really brilliant plan in the long run, more even so if creative didn't think of his motivation before.
Let's not forget that was went Walkers for 'all-powerful' and Bolas was shown to be able to take on any one-on-one. He bested Teferi in his own field of time magic and Leshrac need a trump card from a similar powerful entity to stand a chance against him. But now he, and every other Walker, is mortal, more or less. It's more likely that he figured that now that they are killible flesh and blood, they'd be more likely to team up. So he needed somewhere where he could have a force devoted to him that eh could use as a meat shield. The fact that he could make the Eternals is more of an after the fact benifit