I was playing around with a stark deck built around Gates of Winterfell with the House of Dreams Agenda. The idea being that the deck is 100% characters (and mainly big armies at that) with GoW to bring at least one in per turn. Add the Rob Stark character that brings in Armies for free and plots such as City under Siege to get more out and it could be 3 per turn, every turn. This didn't seem effective enough by itself but what about adding Hellholt Engineer? Would this work okay? I assume you could bang out new characters in the opponents marshalling phase and then reset the Gates with the engineer each time they use a location themselves? Anyone think this would work reliably enough? Any suggestions for making it more effective?

If you play too many expensive characters, your own agenda will completely kill your setup and amount of characters that you can play per turn. I'd guess that even with HoD and Gates of Winterfell you still want to have some amount of 0, 1, 2, 3 drops so that you don't waste your setup on just Gates.

Playing only characters makes your board susceptible to resets since after a reset you won't necessarily have enough gold from your plot to replenish the board (even with the Gates). Also not having any tricks up your sleeve (especially, the ones that do direct kill, direct save or direct steal, blank or discard), you won't be able to deal with the annoying locations/characters that your opponent can potentially get into play.

So if you want to try to make it consistent, I'd still suggest running some reducers/gold producing locations and important events (e.g., narrow escape to save your board from being wiped out with Valar) and you can also go for heralds to search for characters and drop them into play with Gates (making gates more consistent since you can still have a few huge drops and just put them in play with Heralds+Gates combo).

Hellholt out of house will cost you 4 gold OOH and your opponent can easily play around it when he sees your Gates in play, so it can be a dead card. Plus if it gets discarded with Dissension you will be behind your opponent on 4 gold. You can try to figure out a way to abuse steel link (when Maester with steel link kneels, stand a location you control) and some Maester (e.g. Initiate of Citadel to discard 0 attachments from some random character, lol.=)

PS. there are people trying to assemble similar decks on agotcards.org, so you can check out their decklists for ideas and inspiration.

Effects that stand your characters would be good to include as well. NE might be the best choice for a restricted card since you will be so vulnerable to resets.