I don't know what to think about the Splig deck. I too read a lot of good things about it, but when I looked at the cards it just felt too restrictive in my opinion, like Valyndra and her boosts to large monsters. If it suits your playstyle then go ahead, but from my own experience as a OL I would rather opt for something more generic giving me lots of choices without having to consider the type of monster I picked for the quest. Because otherwise I may end up with dead cards in my deck/hand. Yeah I can set them aside when selecting 15 cards, but they're still dead cards I cannot even consider. I have no real answer as for which plot deck would be best to pick, but I'm currently testing both Alric and Merrick in my campaigns and they seem fine. Belthir looked interesting too but I wanted something more than just utility.
Quite frankly it would have been GREAT to pick one school/trait of whatever can be used to classify the agents, and be able to purchase ANY card from any plot deck at a cost resolved around the category it belongs to versus the specialization you chose or whatever the name. The OL cards system is great in that sense. Okay that top tier card requires you to own lower tier cards of the same type, so why not applying the same mechanism to plot decks? I want to be able to buy any agent's level 1 plot card then I can specialize and get more from 2-3 of them. Ultimately, what's the problem with having two possible summon agents in my plot deck, especially since I won't be able to summon that agent in a certain number of quests anyway? Just add a restriction that only one agent can be summoned by quest and let the OL decide which one he/she wishes to use.
I'm in the situation where I just pile up threat tokens and wonder if the system around these couldn't have been better implemented, or at least less restrictive.
Alternatively, getting access to certain plot cards could have been a reward or a special quest event, not that it would put plot cards straight in your hand (or?) but at least make them available for purchase.
Similarly, also because I often get the question from my heroes, why not unlocking class abilities outside of the class you originally picked from the start, so that a Champion may purchase a Knight class ability for either its price (when made available from a quest) or with an increase of cost (based on archetype)?
I just don't see why it should be a hassle to select a class/OL/plot deck and restrict yourself to these few cards for the whole campaign. I think the game is fine as it is, but I also think many of these unnecessary restrictions/boundaries have been set which don't really need to exist at the first place. It doesn't make the game more complicated to allow players to get access to more abilities, it just diversifies the game more. You can still block access to other archetypes or if you think two agent decks are incompatible because they're enemies thematically or whatever.