Yup, I'd go with Wazza300's idea. Depending on the laptop and how it would look (as a lot have a fancy cover over the optical drive) get an optical drive bay caddy. They're about £30 tops. Out comes optical drive (stick it in a USB3 self powered case - about £10) and in goes the slower storage drive.
Keep the SSD as boot and use the rest of the space well (current main game for example) and the storage drive in the caddy for everything else.
The rare times you need the optical you can either switch it back in or if you went the external route just plug it in via USB.
Personally my next laptop is likely to have an mSATA for boot, an SSD for steam folder and 2xslow drives for mass storage (I don't really use a laptop for much beyond the reach of a power socket) with a bigger raid (and desktop, full fat GPU) available via thunderbolt. If I can get the raid to double as a NAS then bonus (probably buy the cheapest possible "box" with thunderbolt port then can plug it in the laptop or plug it into the other box and have it host the drives for network use).
