If I had to put money on it, I'd still pick the Cavs to win the East in 2018. Boston by no means would be the favorite, but a lot can happen between now and next May.
While the Cavs pulled it together by the time the playoffs rolled around in 2017, there's plenty of dysfunction both on and off the court there, and that may only get worse over the coming calendar year. In the wake of losing in the Finals, combined with the internal pressure of LeBron's pending free agency and the (potential) hire of a GM with zero front office experience, the Cavs could easily implode (relatively speaking) this coming season. I'm not saying they'll drop off the face of the earth, but being a 2 or 3 seed and not having enough chemistry to pull it together for another monster Eastern Conference playoff run is certainly possible.
If that happens, a team like Boston, Toronto or Washington could make things interesting. Two huge keys to me are whether or not Jaylen Brown makes the leap this coming year and how good Tatum can be by the time the playoffs roll around. If Brown has truly taken his game to another level, Tatum is a legitimate Rookie of the Year candidate and plays like it in the playoffs, IT is healthy and continues playing at the same level, the rest of our core stays healthy and either improves or at least stays the same, and we've added some rebounding and interior defense (whether from a rookie or a minor offseason or midseason acquisition), I do think the Cavs could be beatable.
Obviously, a lot of "if's" would have to break in our favor, but the good news is that the future would still be bright with the amount of young talent and draft assets we have.