Collect what appeals to you.
Themes will develop on their own. If you buy something now that does not fit into the theme you eventually settle on, you can always sell it or trade it for something that does.
I guess it's just a reflection of my obstinate personality, but I collect things that -I- like, and am not very likely to ask other people to tell me what I should like. Establishing arbitrary boundaries on my
collections also seems to be pointless. Some people like to collect with the idea of getting "the
complete set", while I'm content with representative samples. Some people like to choose one particular era or theme and stay within it, I'm more likely to
work somewhat within a theme, but not feel restricted to it. I also shift
themes easily as time goes by.
Read up on any topics that catch your eye - I often find that the more I know about something, the more I appreciate it. The stories and histories reflected in the coins are the real hooks for me - the coins are only token representations of people, things and events of long ago. An individual coin in my
collection might be common and not particularly attractive, but if it represents something that means something to me, then I enjoy it.
In the end - you are the person who will live with your
collection, will look at it and will appreciate it most. So why not just collect for your own tastes and appreciations? Don't make it an obligation to fulfill someone else's idea of a
collection - have fun with your own choices.