BMW prototype based on Vision EfficientDynamics concept
The "i" obviously comes from BMW's Project i, while the number draws on BMW's usual model-naming convention--and the 8 kindles memories of the fantastic-looking 850ci and its brethren, the E31 8-Series. The name fits, given the VED's coupe layout and overall proportions.
The naming move also presages the names of the rest of the lineup, presumably based around the same core framework: iX, where X is the model series. Could the Megacity Vehicle be the i1? Or possibly i0 (zero positioning it below a 1-Series-based entrant and signifying its electric-only zero tailpipe emissions)?
Using the i8 badge jibes with last week's leaked "i" logo, which reportedly came with registration of the trademarks i1-9 as well, perhaps indicating a different model-naming scheme for the Project i vehicles, but built around the usual BMW nomenclature.