Whenever the Bay Area economy booms, developers flock to downtown Oakland with brash talk of tall towers. Things quiet down. Big plans fade.

This time, though, the hype comes with cranes. Two high-rises are under construction between Broadway, downtown’s main drag, and Lake Merritt. Another seven have been approved or are under review.

But the swelling wave brings its own challenge to Oakland decision makers: how to ensure that tomorrow’s towers don’t just deliver revenue and residents, but also a renewed urban center that’s worthy of the city as a whole.

This isn’t to say that tall, 21st century buildings shouldn’t join the already eclectic cityscape. What’s important is that they enjoy the jostle with the past rather than pretend the local scene does not exist.

The most vivid example of that tension is on the 1100 block of Broadway, where 18 stories of offices would fill a vacant lot and restore the Key System Building, a seven-story classical treasure from 1911 that has been empty since the Loma Prieta earthquake in 1989.

The structure, which has been on the National Register of Historic Places since 1981, starts with a tall base that once held a banking hall. There’s a four-story shaft of yellow brick that rises in a deep U and then two stories of florid terra-cotta punctuated by arches and topped by a snappy cornice. Everything fits together in a way that’s inherently right — the type of big-city structure that was commonplace before World War II but now is all too rare.

The sum of the parts radiates an assured ambience despite decades of neglect and a procession of failed development plans. The 1100 Broadway project would offer long-needed salvation, with the new tower providing seismic support to the original while restoring the masonry details. Inside, the two buildings would form a single unit.

The developer is Ellis Partners, which in March purchased the Key System Building and the land next door. Ellis also bought the site’s entitlements for a tower that was approved in 2008 but never built , and hired the architecture firm Gensler to design a new tower within the allowed space.

In terms of the current proposal, that’s where the trouble begins.

The design submitted to the city by Ellis this spring featured a block-long slab broken into eight-story stacks of glass set one atop another. Some were skinned in multicolored sunshades, some weren’t. Some extended 5 feet beyond the sidewalk, some didn’t. The final cube at the top cantilevered out 25 feet over the Key System Building.

In the abstract, as a mainstream version of futuristic high-rise blocks, the textured grid had a certain blunt drama. But it was all wrong for the location, an architectural interloper showing us who was boss.

After a chilly reception from planners and several public hearings, the off-center stacks have become a more conventional slab. Sunshades are deployed to give an illusion of verticality. The upper floors no longer extend beyond the property line. The two-story void below the cantilever has been filled in, so that the addition rises directly from the Key System Building’s roof.

But the priority remains the same: to pack in as much space as possible, as efficiently as possible.

Even in the architectural renderings — the most flattering venue imaginable — the old is belittled by the new. One example? The 1100 Broadway slab pulls back above the second floor to reveal the northeast edge of the landmark, where the cornice turns the corner. But the setback is only 5 feet, cutting off the historic finale before it concludes.

Relatively simple changes would sharpen the tower and allow the Key System Building to breathe. For instance, the entitled 2008 project was 20 stories but the same square footage. One of those missing levels could be restored: The extra height would allow space to deepen the setback alongside the Key Building on Broadway so that all of the cornice remains, and pull back the addition so it doesn’t loom over the Key System Building quite so much.

The catch? This would be a bit more work and a bit more money. It also would mean slightly smaller floor plates, which supposedly are less attractive to the tenants that Ellis hopes to land. That’s why the developer lopped off the upper floors and overlapped the Key System Building to begin with.

The current version is an improvement, no question. The staff report on 1100 Broadway published last week recommends to Oakland planners that it be approved. The commission is likely to vote that things are good enough.

Grading on the curve, as it were.

Whether or not 1100 Broadway goes forward as now designed, Oakland in the future needs to keep pushing developers to treat the downtown landscape with more respect.

Architects know the responsibility they have: “Towers shouldn’t feel like a UFO that landed on the city, or call too much attention to themselves,” says Chris Pemberton of Solomon Cordwell Buenz, an architecture firm doing several Oakland high-rises. But those clients run the numbers in a city where construction costs are similar to San Francisco while the likely rents aren’t as high. In Pemberton’s words, “It’s a constraint from the budget side.”

This translates to developers wanting towers that fit formulaic expectations. And Oakland, understandably, likes the idea of replacing vacant land and parking lots with buildings filled with newcomers who live or work downtown.

But the downtown also is an attractive, transit-friendly district that will only become more appealing with time.

“Even within a cost constraint, there are choices a developer can make,” says William Gilchrist, Oakland’s new planning director. “You want a sense of continuity. That’s what keeps things alive and engaged.”

Oakland is fortunate to have a generation of downtown landmarks in all architectural styles, from the Key Building and the old-school Tribune Tower to 1970’s Ordway Building, 28 stories of crisp aluminum. Developers and architects should be prodded to live up to those standards — if only to show that Oakland’s future can be as memorable as its past.

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron