while being a firm Atheist myself thought it would be more fun to try and argue (not even sarcastically!) for the existence of God.
I'm not an atheist, but I tend to do the same thing when arguing with my friends. It does add more substance to the arguments, and definitely helps one to open his/her mind and either reinforce his/her beliefs, or completely change them.
I'm sorry but you're just discrediting human intelligence/our understanding of the universe (so far) by pretty much saying that without a god, everything would just be random garbage floating around. Yes the universe is complex and beautiful, that doesn't mean it was created by some omnipotent being. Unless you're using a different term for God, since no one knows what God truly is.
Disregard what type of God he's talking about - it could be anything. In any case, he's not talking about beauty and complexity. Beauty and complexity can refer to, for example, the Grand Canyon, and we all know that that can form by natural laws.
Signs of intelligence (such as the DNA molecule, a code/language, much like binary) do NOT form from natural laws to our current scientific knowledge. To say hence it comes from God is committing a 'God of the Gaps' fallacy. But to say it is evidence of a higher intelligent being is not. Notice the word 'evidence', not proof. Evidence can easily be denied, but to our current scientific knowledge, this evidence only appears to be able to come from an intelligent mind. To say that one day science will debunk it is committing a 'Science of the Gaps' fallacy.
Again, why discredit human intelligence and the beauty of the world?
We shouldn't discredit human intelligence. But we shouldn't also be an arrogant species that believes everything has an answer. We cannot know everything - that's a fact. Just because evolution currently cannot explain the immune system, doesn't mean that we have to fill that hole with God. And just because it cannot explain it NOW, doesn't mean we have to fill that hole with 'science will one day find a reason'. There's nothing wrong with saying, 'I don't know', unless you have good evidence for what you believe. Don't get me started on dark matter.
Why would an omnipotent being need for us to submit to him/her and follow some useless guidelines in order for us to continue in the "after life"? I consider myself to be a pretty good person. I have code of ethics and a good moral code. Am I going to hell for not submitting to a certain religion? If so, I wouldn't want to worship that being anyway.
This is starting to pertain to religion, which is not something I will discuss, as I am arguing for any God (call it whatever you want), not necessarily the God of religions. However, imagine you were to create a new race of robots. You would likely consider yourself the father of these robots. Now imagine these robots turn on you, and don't believe in you, don't believe in things you say, etc. You'd be pretty disappointed, no?