Cursed arti spawn. Leave doom as it is, so if you wanna bless one, you gotta work for it. Also along the same lines, turn ToT back on.
I say all old fel dungeons have cursed arti drop like tot. Give people a new area to hunt.
Second character slot. At this point it doesn't matter in the slightest, what with soulstones losing their delay. Non siegers have asked for this, maybe it would draw some people in.
Do something about faction arties. I say either get rid of them altogether, or don't tie them to individuals (just to factions in general so you could loot and use if you were in any faction), and let anyone farm silver, and have a turn in for silver of faction arties someplace for faction as well as non faction people, that way faction people will still have easy access to items, but the non factioners will have a part to play in the goings on.
Better source of relic frags. I don't know where. Drop them on any abyss creatures, with some rarity. Thats the main problem with imbuing on siege. You gotta do bods to get frags, which is obnoxious.
I think those are the best choices.
Extra resources, meh, its not needed, but would be fine. I think they are plenty easy to get as it is.
As for skill gain, I think we are sitting pretty good right now. I am .4 away from finishing a character I thought was gonna take months more. And thats without the benefit of sots and alac, which should be much much easier to get from this point.