Hmm. Isn't the current module set about killing a goblin king or something? Isn't it practical to limit the gameplay to what's there now, i.e. dealing with the goblin king? No overland? And why are subraces important?
Not saying that this is how it should be or not, just curious of thoughts on the above.
Ah, I think Zireal & I may have misinterpreted your question as 'what would be necessary to turn Incursion into a 1.0 version of the full "Return of the Forsaken" planned game'.
In terms of what would be needed to get to a 1.0 just for the "Halls of the Goblin King" module, I think it's mostly there already - you can get down to the bottom of the dungeon, kill the king & emerge victorious, which is essentially the game. In terms of buggy-ness, I haven't played a full game recently, but I think most of the main showstoppers have been addressed?
The extra prestige classes / subraces / psionics aren't really
necessary to finish the game successfully, so would probably come under bonus content.
The level cap could be pushed out to 20 & the associated feats/spells added; that was the planned level cap for the full game, but might be overkill for dealing with the Goblin Dungeon as is (though I imagine the dungeon depth could be increased fairly easily to compensate). That'd be quite an undertaking, though it's more adding lots of content rather than the adding new code features work that the overland etc. would involve. And again, none of that is really necessary to complete the game as is.
The full "Return of the Forsaken" game was planned to be a lot larger scale than just the one dungeon, more ADOM / Omega scale, hence the features we mentioned (and I think the Goblin King's dungeon itself probably would be a lot smaller & consistently goblin-themed in the full game; in the dungeon variety comments I added earlier Journeyman mentioned that it's a bit of a hodge-podge of all possible terrain / monster types at the moment mainly because there's nowhere else to put them)
And on the gaze attacks, I think I'd echo Gomissar's comments - a lot of the gaze attacks are very very nasty - I'm thinking of the grey nymph's "instant death due to beauty overload", which wouldn't be a classical gaze attack - if averting your eyes / fighting blind doesn't work against those, then that will make them a *lot* more deadly for fighters (/anyone who doesn't have access to a death ward spell), and I can't really think of how you'd deal with those sorts of creatures otherwise