What is happiness and fun? In the context of EtG?
This varies from player to player. The ability for EtG to reward many different player types is one of its greatest aspects. Here are a few stereotypes:
The Collector has a goal and gets satisfaction from growing their card pool towards that goal. This varies from the 12 of each card players to the 4 of a subset of rares player. These players tend to have a love hate relationship with the spins.
The Warrior enjoys the thrill of conflict. Facing a worthy opponent is fun. The definition of worthy varies.
The Artist enjoys creating. This varies in medium from Decks, Card Art (Vrt), Card Ideas and even Lore.
Players may fall in more than one category or might fit in unmentioned categories.
Do all meta games eventually devolve into rock paper scissors or rock paper scissors lizard spork? Is this bad?
No. Some metagames are rock rock rock.
You know of Rock Paper Scissors can be expanded further (see RPS 25).*
It can be modified further by adding a dimension. (Each 3 consecutive options in Rock Paper Scissors Lizard Spock is its own RPS)
Imagine a RPS with 100 options and 5 dimensions.
Now change the certain victory to a probably victory (Lizard has a greater chance of beating Spock than Paper does but Spock could still win)
This is what I would call a mature metagame.
The only objection I see to a mature metagame is that it is too complex to hold in my head.
Would EtG be better with a much larger card base or a small one?
Larger. More options create more diverse experiences and thus more wealth(in the form of happiness).
How do you place more emphasis on creativity/deck building in EtG without grinding or forum based PvP?
Expand PvP Duel. Let people play PvP Duel without prior arrangement (a lobby) and have it separated based on intent (Competitive decks, Creative decks).