no mention of Onslaught at all, no tease really, Xavier doesn't go ape shit on Magento, just talks to him in his mind for the whole episode, doesn't bottle Magneto's hatred into himself like in the comics. So if you did this, it's going to be really weird pulling Onslaught out and saying, "Remember the finale from last season? We didn't show you, but this entity was created when Xavier did that." Overall Bastion, great, Cyclops, great, Jean Grey Phoenix, kinda outta nowhere and then leaves shortly after it saves her and the team for plot armor sakes.
Spoiler!
That is pretty much what the comics did though. There wasn't anything in Fatal Attractions setting up Onslaught save for one line, that could be read either way, about Xavier taking Magneto's hatred away. At the time it was mostly interpreted as Xavier erased it. It wasn't revealed until after Onslaught was introduced nearly 3 years later that Xavier meant that he literally took it into himself.
Anyway, I do agree about the Phoenix thing. It's maybe the only flaw for me. It almost felt like they didn't see the plot hole until it was already animated and tried to throw in a line of dialog to cover it up. It would have helped if they had even come up with a slightly better explanation, like Jean had to let the Phoenix go before it took her over or something. But they basically just had Jean go "LOL the Phoenix left".