The Exynos 8890 is still an 4+4 big.LITTLE design using four Cortex A53 cores as the little cores, but for the first time we see Samsung's custom developed CPU architecture deployed in silicon. The new core, officially called the Exynos M1, is the first fruit of years-long efforts by Samsung's Austin R&D Center to try to create an in-house CPU architecture. What we do know of the M1 is that it's still very similar to ARM's big core architectures (And thus might be a derivative) such as the A72: It's still a 3-wide OoO design with the same amount of execution pipelines and similar, although not quite identical pipeline stages on the execution units.
Samsung is claiming the Exynos 8890 will provide up to 30% higher performance and 10% better power efficiency than the Exynos 7420's - although the wording is a bit vague and doesn't specify if we're talking about a pure architectural comparison or actual implementation comparison, as previous PR numbers on the Exynos 7420 also didn't quite represent the full improvements of the chipset.
Samsung follows MediaTek's example by dropping the use of ARM's CCI IP in favour of designing their own cache-coherent interconnect fabric aptly named SCI (Samsung Coherent Interconnect). It seems that vendors are keen to try to improve their SoC architectures by designing fully optimized SoC fabric solutions and I guess Samsung saw the need to differentiate in this regard.
On the GPU side, we see usage of an ARM Mali T880M12. This is the biggest Mali core implementation to date and increases the number of cores by 50% compared to the Exynos 7420's MP8 configuration. Keeping in mind that the T880 also increases ALU pipelines per core by 50%, we're looking at an 2.25x increase in computational power assuming Samsung kept the clock frequencies equal. Alternatively, they could go lower in frequency for much improved power efficiency. Samsung advertises 4K as an option for this SoC so likely we're looking at a very powerful GPU setup.