You MAKE the good candidates by how you treat the potential bonsai while growing them on. You might look into reliable information provided by a well known bonsai practitioner (name of Julian Adams). He has written a LOT about how to treat and prepare the stock you'll be growing on for future use.
This image ^^^ was taken in early spring. It shows, on the day they were lifted, 4 trees that had been growing on in raised beds devoted to potential bonsai. They have had nothing done to them other than lifting them each spring to prune roots and rotate the tree 90* to promote more even branching. When the trees were just out of the whip stage, they were wired to provide some movement (curves) in the trunk (of some) to make them moire interesting. You could say that this is the first day of their journey to becoming bonsai. I almost ALWAYS wire trees going into the ground so they have interesting trunk lines, and often plan chops and the position of branches long before the tree actually grows them. The planning part is very strategic. Pruning and wiring is my favorite thing to do. Repotting is my least favorite because it's a hurry hurry high pressure period each spring, then again in early summer when tropicals get repotted.
Never be in too big of a hurry to discard a tree. I've had trees for years that I didn't know what to do with, when suddenly an inspiration comes and it ends up being a highly favored tree. Perceived flaws can become important focal points because there is a great deal of beauty in certain types of disorganization and planned asymmetry. I know one thing, unless you rise to the pinnacle of perfection, you'll never want for a challenge.
Best luck! If you have questions, I'll help all I can.
Al
