It was a shame that I was not able to enter an earlier phase of the initiative. But there's still time.
I'm not going to make major observations about the previous phases, since update #2 is actually closer to the ideas I had when I started reading about the initiative.
I began the stylistic transformation into vintage sfi-fi atmosphere.
That would be my first suggestion, I think it will present a very different and innovative idea, very interesting.
The BTC is lit with warm light that could be made with real golden leafs in the final canvas artwork.
There is also the "15" for our anniversary and below nicknames of Satoshi and Theymos, two of the big names that could be named there as pillars of BTCtalk.
Looking at your sketch, which looks good to me, I would make the following suggestion:
On the left side of the tower, you have the forum timeline. On the right side, you could create another timeline, linked to mining.
Every 4 years, the reward decreases, until the last new BTC mined. So, there could be some kind of column/tube or like an elevator outside the tower that is going up to the last mined "sat".
I don't know if in 5 years' time you're thinking about creating new art for your 20s. But this element could be a dynamic element that would accompany future arts.
Now, it was time to figure out where this "timeline" would fit, whether in the back plane or the middle plane.
1.) How to visualize a spammer?
2.) How to visualize merit?
3.) Who to paint as main character in the front right (if anyone)?
1) A spammer would be like a beggar. Not just one, but a group of beggars around a drum to gain access. This bottle was "SPAM".
2) I think you could create something like a merit store. Let's just say it was the gadget guy who sells all kinds of trinkets. And next to him, we would see someone disruptive who would simulate merit.
3) It would be "Master Yoda", but in this case Satoshi. He watched everything from under his cloak, but no one could watch him. Maybe too much Star Wars... sorry.