According to a report on NordicHardware, MSI is holding off building a Maxwell-based 'Lightning' series graphics card. Supposedly, MSI is doing so because it expects an even higher-performing Maxwell part to arrive in the near future.
The Lightning series cards from MSI are the company's extreme overclocking-oriented cards which come with extremely hefty and powerful coolers, as well as very elaborate PCB designs carrying a rather generous number of VRM phases. As a cherry on top, the GPUs are also hand-picked for more stable overclocking.
In the past, the mainstream parts carried a dual-fan cooler that closely resembled the standard TwinFrozr cooler featured on the "Gaming"-branded cards. However, the GTX 780 and GTX 780 Ti that were based on the more power-hungry GK110 GPU got a new Lightning cooler, called the TriFrozr, which featured three fans and a ludicrously thick heatsink.
So, taking that history into account, we reason that the lower TDPs of the GTX 970 and the GTX 980 don't allow MSI to justify building a special overclocking-oriented card yet, as the "Gaming"-branded cards already overclock quite well.
MSI has confirmed that at the time of writing it has no plans for a Lightning card on Maxwell due to restrictions that Nvidia has placed on overclocking.
Might there be a more power-hungry GPU on the way? Perhaps the GM200 or a GM210? MSI claims they have nothing on the product lineup, but that doesn't rule out the possibility.