The Lakers, if they acquire Dwight Howard, figure to pose a major obstacle for the Heat next season in Miami’s quest to repeat as NBA champion.

But long-term, another potential impediment looms — not a team, but a document.

The collective bargaining agreement ratified last December will create serious challenges, depending in part on owner Micky Arison’s willingness to pay a luxury tax bill that could top $20 million or more annually in a few years.

If the Heat wins the next championship or two, it would be surprising if the Big 3 is broken up in two years, considering Arison’s desire to win. But it’s not out of the question, considering the luxury tax becomes far more punitive beginning with the 2013-14 season.

And keep in mind that LeBron James, Dwyane Wade and Chris Bosh can opt out of their six-year contracts after 2013-14 or 2014-15.

If the Big 3 stays intact, the luxury tax could seriously impact how the Heat constructs its supporting cast. And rules involving midlevel exceptions also will limit how the Heat can fortify its roster as long as it retains three contracts at or near the maximum salary.

The Heat was one of six teams required to pay a luxury tax for this past season, with Miami owing $6.1 million, or $1 for each $1 spent over the $70.3 million threshold.

But beginning with the 2013-14 season, teams that are between $1 and $5 million over the tax threshold must pay $1.50 on every $1 above the threshold. Teams that are $5 million to $10 million above the threshold must pay $1.75 for every $1.

And teams that are taxpayers in four out of any five seasons (starting in 2011-12) must increase their payment by $1 for each dollar spent, which would be financially painful for any owner.

Heat president Pat Riley said Arison “loves winning championships, but there is also a limit, and we have to be very conscious of that. We haven’t had long discussions about the luxury tax. Micky has always had the opinion, you give him the right name and that right name can lead this team to the promised land, he has always said yes.

“With how punitive the luxury tax is, and not only that, but the bite he has to take from a revenue sharing standpoint, that has to be considered, but that’s going to be his decision.”

For 2013-14, when the luxury tax threshold is projected to be in the $73 million range, Miami would have $85.6 million in cap commitments if Ray Allen and Rashard Lewis don’t exercise opt-out clauses; if the team exercises a $4 million option on Mario Chalmers; and if the amnesty clause is not used. And that’s before the Heat even considers using its $3 million taxpayer’s midlevel exception.

Amnestying Mike Miller and eliminating his $6.2 million salary for 2013-14 would save the Heat as much as $10.8 million in taxes that year and slightly more the next, making it highly tempting.

Each team can amnesty only one player during the length of the new 10-year labor agreement but can only use it on a player signed before the deal was ratified last December.

Two summers from now (after the 2013-14 season), James, Wade and Bosh all have opt-out clauses, with James and Bosh set to earn $20.6 million and Wade $20 million for 2014-15 if they do not opt out.

How much more, if anything, could they make if they opt out? That’s undetermined and will remain so until the summer of 2014. Cal-Irvine computer scientist and ESPN contributor Larry Coon, considered the foremost expert on the NBA salary cap, explained it this way: