Ready for another round of airline mergers? Wall Street is. An influential airline analyst is stirring the pot with a note positing a combination of United Continental and JetBlue Airways, one of the upstart carriers that have reignited competition on some key routes. They're an unwelcome irritant to "legacy carriers" United, American and Delta, which went through a series of megamergers in order to snuff out profit-crushing fare wars that helped force all of them into bankruptcy reorganization since the turn of the century.

Merger integration was especially painful for Chicago-based United, which endured years of glitches as it struggled to combine operations with Houston-based Continental Airlines. Nevertheless, analyst Hunter Keay of Wolfe Research lays out a case for buying JetBlue. "Though airlines have consolidated to the point where most assume further M&A is unlikely, we still see the possibility of UAL going after JBLU in order for UAL to address its own perceived lack of a sufficient domestic presence," Keay writes.

Keay points out that JetBlue would give United more capacity in the U.S., a priority for CEO Oscar Munoz and top lieutenant Scott Kirby. United has been adding service on routes connecting smaller U.S. cities to its major domestic hubs. Yet as Keay notes, American and Delta both have more domestic capacity than United—American significantly more. Adding JetBlue would give United a slight edge on American in domestic "available seat miles," a measure of airline capacity.

There may be other reasons to buy JetBlue. Acquiring a troublesome new challenger would eliminate one source of growing price pressure across the industry. United also might want to keep JetBlue out of the hands of another rival, such as American. "United has the opportunity to strike at an opportune time," Keay argues, noting JetBlue's stock is down, United's balance sheet is strong and low interest rates reduce deal financing costs. JetBlue declines to comment, and United didn't respond to my inquiry.

Keay devotes considerable ink to addressing antitrust obstacles to a deal. Federal antitrust enforcers greenlighted the last round of airline combinations in part because of concerns that some carriers wouldn't survive alone. But with consolidated airlines now enjoying profits so robust that American CEO Doug Parker recently said he doesn't think his company will ever "lose money again," another round of mergers might not get such a solicitous regulatory reception.

A United/JetBlue tie-up would raise serious competitive concerns in the politically sensitive New York market, where they have a combined 42 percent market share. Among other possible solutions, Keay suggests regulators might agree to consider Newark Airport, where United dominates, as a separate market from LaGuardia and Kennedy airports, where it has a smaller presence. He also raises the possibility of lenient antitrust reviews under business-friendly Trump appointees. Maybe, but political heavyweights like U.S. Sen. Chuck Schumer, D-N.Y., and New York Attorney General Eric Schneiderman aren't likely to sit still for a deal that gives a single airline nearly half their home market.

At a minimum, antitrust regulators would require significant route divestitures that erode the value of the deal to United. And the benefits of acquiring JetBlue may not be as great as they initially appear. Keay notes that only 16 percent of JetBlue's capacity feeds directly into United's domestic hubs.

It seems to me that United is better off strategically adding flights on its own, in line with Kirby's plan to maximize traffic through domestic hubs. Buying JetBlue would bring complex merger integration challenges—probably including a serious culture clash—threatening United's recent rebound from customer-service and on-time performance woes caused in no small part by its last big deal.

But like it or not, the scenario is now on the table. And CEOs don't need much prodding to chase acquisitions. If consolidation takes off again, no airline boss will want to be left at the gate.