I can think of a few things that may give you direction:
Your first problem is to identify the audience, and with it, the exact goal of the aesthetic design. Why does your client wants something futuristic? What is the client trying to accomplish? Who is the audience --who does he want to impress? Could it be potential users (i.e., to sell to them or to recruit them)? Superiors? Visiting dignitaries? The client him/herself? That will give you direction on both the aesthetic style to pursue and how to mesh it with usability. For example, it doesn’t matter if you think it’s cheesy. What matters is if the audience thinks it’s cheesy. If the users aren’t the audience, then you only have to avoid them reacting negatively to the aesthetic. Beyond that, they probably don’t care. You can research and test how the audience and users react.
As for usability, depending on who your audience is, there’s a good chance that it gets only a brief, superficial exposure to the app. After a short period, the audience has moved on to look at something else other than the app (e.g., the demo is over). Even if the audience is the users, its members may soon be so engrossed in reading and/or using the app, they’re no longer noticing the aesthetic anymore –they’ll be attending to the content, not its form.
If you only have to concern yourself with the first brief exposure to the audience, then you can achieve the aesthetic without compromising usability pretty easily, because the most extreme graphic design would need to appear only when the app first starts –maybe on the dashboard/home page, or perhaps just on the splash screen. Beyond that, the “futuristic” design is confined to mostly color, shape, and font choice that’s subtly reminds the viewer of the strongest images in the beginning. Things like subtle color choices (e.g., choosing dark green versus black font) will have little impact on usability except under the most extreme use conditions.
Don’t Look Futuristic. Look Like the Future
Once you identified the audience, you need to arrive at the specific aesthetic design. Your problem may be that in drawing inspiration from movies and computer games, you’re imitating a fictional vision of the future. By imitating something make-believe, your app looks make-believe –the audience associates the aesthetic with something trivial, not a dead-serious application to be used all day. That may be why it looks cheesy.
The solution is to look at real things for inspiration. What does your audience associate with the future? The F-35 fighter? The ISS? The Jaguar supercomputer? Exactly what you choose depends on what sort of future you want to convey. For example, is it the competitiveness of the F-35, or the cooperativeness of the ISS?
You probably want your aesthetic design to evoke the functional design of your source of inspiration. For example, your dividing lines in the app could resemble the stealth-enhancing zig-zag lines of the F-35 landing gear doors. You probably don’t want to copy the aesthetic design from something else, even something real (e.g., the mural on the Jaguar). Like making a Xerox of a Xerox, you’re not going to come off as fresh and solid as the original. You want to be original yourself. By definition, anything already out there isn’t the future.
Don’t Look Like the Future. Be the Future
Maybe this isn’t about aesthetics at all, but about creating a new way of interacting with the app. You say your users will be heavy users of the apps. That means it’s worthwhile for them to learn an innovative user interface if that interface provides superior performance once the user has learned it. Maybe what your client really wants is an innovative control, or interface idiom, or communication channel that makes the app better for your users.
Maybe you can make your users more productive with integrated voice-and-gesture interaction. Maybe you need to use stereophonic sound to orient the user to items in physical space. Maybe it’s time for a head-mounted display. The exact innovation will depend on your user’s tasks and the technology available to address their issues. Study the users’ tasks for problems to solve, and study the HCI literature and other technologies for possible solutions.
This is the other way to mesh usability and aesthetics, where improving usability with innovation is the aesthetic.