Well if we had better drivers for GFX cards then we could run nice games like this, but I'm sure AMD, IBM and other's new open source policy only pertains to Linux groups. :/
Anyhow, I'd say Dark Forces 2: Jedi Knight if it was open source.
A nice port of DOSBOX is important, that would give us Dark Forces 1 and some other classics.
If we had a decent port of PrBOOM we could run my upcoming Nintendo DS star was project (made for prBOOM on DS, but will work on PC), and if we had GzDOOM we could eventually see my hi-res release of that on AROS some day, but those days will never come.