Presumably he wanted a shitload of money to spend last summer and they said no so he quit, can't imagine he's coming back without the promise of huge transfer spending. Could be the ultimate power play, could be a disaster.
Even if he spends £500m this summer I don't see it going well.
People get angry when I say this, but Zidane's Real were not very good. They were actually pretty terrible for long stretches and relied on probably the greatest "big game player" of modern history to bail them out. They were on the verge of getting knocked out of the CL several times each season before Ronaldo pulled something out of the hat.
Zidane had a team of world class players + Ronaldo, and he couldn't make them play well over a league campaign. Replacing the world class players isn't going to change much, just add a ton more pressure and disappointment.