Finstar :
A worthwhile upgrade would be an i5 8600k or a Ryzen 5 1600.
Yeah, it's important to realize the 8th gen i5s are hexacores - 6 cores instead of 4. Frankly, they're better than a 7th gen i7 except for single-threaded tasks. And given that the i5-8400 (which boosts to 3.8-4.0 GHz) costs $10
less than the i5-7500 (which boosts to 3.8 GHz), don't even bother with the 7th gen.
That said, CPUs haven't improved much in performance since Sandy Bridge. Unless you want some new feature that's been introduced since Haswell (lower power consumption, 6 cores, USB 3.1, better integrated GPU, streaming encrypted 4k without a dGPU), there's very little reason to upgrade.