Howard would dramatically improve our defense (a HUGE improvement in defensive rebounds and blocking, not to mention making things a whole lot more crowded in the paint). He would also have a significant impact on offense, particularly with regards to offensive rebounds and put-backs. We have shooting troubles, so getting those second shot opportunities and fixing missed opportunities with put-backs would make a difference. I think Brad Stevens can find a way to get Howard limited-but-key shooting opportunities where the probability of him being fouled AND getting the basket is high. I am fine with Howard being fouled all of the time so long as most of those fouls result in three-point opportunities. There has got to be an analytics-driven way to put Howard in that position...
Given his decline in scoring, along with injury concerns, I do not expect he will get the blockbuster long-term contract he wants unless he is willing to go to a small market, so I think he may bite on a shorter contract in a big market with a championship-caliber coach and front office.
Howard can clearly be the number two guy on a championship team. IT can clearly be that number three guy. Alas, who will be our number one guy? Simmons or Ingram? Perhaps, but one would expect that to happen at the earliest in their third or fourth year in the league, by which time it may be too late for Howard...
That said, bringing in Howard and another rebounding/defensive center (a must since we need to avoid a huge drop-off whenever Howard sits) to replace Sullinger and Zeller would almost certainly make us the top contender in the East this side of Cleveland. With luck in the injury-department and key injuries to Cleveland and the winner of the Western Conference, we could even have a shot at the championship...
Getting back to options A, B, C, ..., Z: Obviously option A would be to sign Durant since he is arguably the second or third best player in the league. However, our top need is clearly at center, so I suspect that getting a quality center is a part of option A, B, C, ... Lo and behold, Howard appears to be headed towards free agency, so he will be the second best UFA center available (the edge goes to Whiteside given his relative youth, but the dude is definitely getting a max contract, may not be worth it, and does anybody really see him leaving South Beach? Drummond is even better, but he is a RFA and it would be VERY surprising to see the Pistons pass on him).