Sometimes when I feel like going back and playing Myth solo (i,ii or iii) I'll see if i can give myself a new objective that "you're not supposed to be able to do". Such as wiping out all of the enemy in the city in the river of blood level (using the Fallen Lords plugin in Myth II). It's possible, but takes a very long time! I just worked at trying to save one of the berserks from the beginning of Forest Heart (where the trow and Myrmidons kill them all). I managed to save one and keep him alive the whole level - but that's all. If you're looking for a challenge, see if you can save more than one. Not easy! Or suggest any other "impossible" challenges...
While that may be a difficult and outright frustrating challenge, he did post in the myth 2 board so would assume he's looking for myth 2 related challenges, possibly one with one of your plugins would be suitable?
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It's not the winning. It's the taking apart
I've actually beaten Myth III on legendary... but only by heavy use of 1/2 speed and exploiting the AI weaknesses! The worst is herding those damn ogres, until you learn to build fences with well behaved ogres.