I am on record saying that the Celtics need another superstar to seriously compete for an NBA championship. I think acquiring someone like Butler or George could possibly do that, but the problem with that is that the price to pay would be prohibitive. Especially now that they have the #1 pick.
Furthermore, with LeBron showing no signs of slowing down, and the Warriors being an uber-team, I think it would more likely than not be fools gold to go after Butler or George in the hopes that they would elevate the Celtics higher than either the Cavs or Warriors.
The C's are doing this the right way. That trade with Brooklyn...holy smokes. I think their realistic window for being the best team in the NBA is about 4-5 years away. That team will hopefully have, as the core guys, Fultz, Brown, next year's Brooklyn pick (maybe 7-footer DeAndre Ayton?), Zizic, Yabusele, Smart, and Rozier. Probably a few guys still left over from the current C's as well.
In the meanwhile, the Celtics obviously are very, very good, and could get better (even if not quite on Golden State or Cleveland's level) by adding Hayward through free agency. They'd likely have to trade some current guys (Bradley, Crowder, Olynyk) to make the money work. But bring in Hayward, draft Fultz, bring Zizic and Yabusele to the Celtics (if they're ready to contribute), and next year's team, led by IT and Hayward as the two stars (plus Horford if you consider him to be a star), would be outstanding (IT, Smart, Fultz, Rozier, Brown, Yabusele, Zizic, Hayward, Horford....that would be a very good group I think). But really, it would be a transition to that next team where the young stars would all be peaking just as the LeBron Cavs and the Curry/Durant Warriors start their decline, thus opening the real window, which likely would remain open for many, many years.