This is a very wide question and will draw much opionated answers. But it is interesting to note that Berserk is not just your run of mill manga. I'll try to keep my answer limited to the Golden Age Arc (Spoilers for Golden Age thus are inevitable) and try to reference an Interview I'd read from Miura - Berserk artist Kentaro Miura interview. Also note, The links provided may not be spoiler free, but are here as source and additional reading RELATED to the answer but not a answer to the question.
Inspirations: Berserk in itself has many concious and sub-conscious inspirations,
I was a manga reader. There are things that I’ve consciously borrowed from, but there are also things that have sunk to the bottom of my consciousness and pop up out of nowhere later. They’ve become part of me. Violence Jack and Guin Saga are things I was obviously really into, and I do think that Guin Saga was the biggest source for this fantasy universe. That atmosphere it has just stuck with me and now I think of it as the standard to measure things against, so I suppose you’re right.
In that interview Miura mentions his high school life and themes of friendship and goals in life.
I don’t know what relationships between boys these days are like, but back in the eighties, boys were really obsessed with stuff like how good their friends were at things, how highly they “ranked” in comparison to their friends, etc. For boys, friendship isn’t about consoling each other. Sometimes you even try to take the other guy down a peg or two. But to break away from those friends would feel like admitting defeat, and you do help each other when you find some sort of goal. That’s where the Band of the Hawk comes from.
Going from there, the major theme in Berserk is about your dreams. The complete Golden Age Arc is about the purpose of life, fulfilling your dream and price of achieving them.
Guts and Griffith are polar opposites of each other, but draw close enough for Griffith to make him privy to details he had never told anyone, his dreams. These dreams put a rift in Guts own idealogical framework leading him to leave the band. This reversal of roles from the start of series leads to Griffith's downfall. After his rescue, he decides the "price of achieving his dreams" and activates the King's Behelit.
What we see is
- Quality of life: If you don’t pursue your dreams, then you live the mediocre life, being more susceptible to become affected by those that are ambitious, but it’s safer and less risky. (Peasants, villagers, small-role characters)
- Purpose in life : Guts realization that Grifith's dream doesn't give his life the purpose he wants drives the interlude of the first arc. He leaves the band to find his dream
- However, it also points to what is really important in life. To complete your dream you shouldn't lose sight of whats truly important. Minor Spoilers.
Guts dream is to kill Griffith, but in his anger he leaves Casca alone who is the one person above all he values. This is covered in the future arcs and the 2016 Anime. He also gains new comrades (kinda).
- Griffith lost sigh of this and in the end lost himself. He later sacrificed everyone in order to achieve his dream. The ending shows that the dreams/desires can corrupt even the best of men.
There are other minor themes that come into play are
- Berserk and Friedrich Nietzsche: This is something that is well documented. Miura's Berserk is very similar to Nietzsche's philosphy.
- Struggler Skull Knight's name for Guts is very apt. It is a commentary on the Nietzsche philosphy above. How guts struggles and survives.
- Religion. We see many religious symbolism and analogies in Berserk. https://www.reddit.com/r/Berserk/comments/1wvqit/thoughts_on_religious_analogies_in_berserk/
- War and History: Berserk is a fantasy manga with Historical overtones (set in a medieval like period) But it does provide an insight into how wars were fought. A little of tactics, diplomacy, motivations of different individuals and the human nature itself.
- Fate/Free will: Are we bound by fate or we have our own destiny in our hands. Are there forces dictating the flow of our lives?
- Idea of Evil: Evil comes from inside or outside? Godhand, Behelits etc.