Off the top of my head a few:
1) Age of Empires 2 (gold) - played the first and all the rest, but missed out two, and considering I like RTS and all the rave reviews it gets its a missing hole in my collection I'd like to experience and fill.
2) Homeworld Cataclysm - I've got the whole series, but for some reason I've never been able to get Cat to work. I'd probably grab it on GoG on the hope that they get it working or at least for a revival of interest and some tech help. One of the best in the series, esp since it moved onto a fully mobile base of operations instead of mostly static (that and the story actually built on from the first unlike HW2 which jumped into the middle of another storyline without much linking to the first).
3) Jade Cocoon - ok so they'd have to start doing PSONE converts but I'd totally get this! One of the best games ever and sadly my own copy has degraded beyond playability (that and the PS2 has also decided to break itself....)
4) Battlezone2 - a quirky novel idea for a game and one that has not really been tried again that often (esp in single player).
5) That game with the shark gun by the people that made Kabuto - I've totally forgotten its name - but that game just because its more of the Kabuto crazy madess kind.
That's mostly it that I can recall, one or two other titles might grab my interest for an early buy, but those would be the dead certain ones to get purchased, even if they were each at the £9.99 price point
