From what I've seen over the years most of the figure focus collections that are started by people "just because" tend to end up being sold off just as quickly as they are put together. While it's not essential, I definitely think the key to being able to continually collect items for a focus collection over a period of years (as opposed to mere months) is to actually have some kind of attachment to the figure.
Childhood favourite, sole survivor of your childhood collection or just a figure you could never get back in the day - whatever the reason, it will help immensely if that figure has some meaning, rather than just being one you think is cool (what if your opinion changes?) or because it's not been done before.
Ultimately if a figure focus it's just something fun you've thought of and have no intention of doing it for more than a few months then ignore all of that