As others have already pointed out, things that depend on "fast-twitch" muscle fiber - throwing velocity, vertical leap, sprint speed - have a genetic limit component in humans. We can work to the end range of that limit, but there aren't too many realistic ways (legal and illegal, mind you) that you can exceed them. It's just physiology.
However, there are plenty of training methodologies that will take someone to that limit. Increasing strength is likely the best place to start for a relatively untrained athlete.
I'd read what Kelly Baggett has to say about this topic from Higher Faster Sports, as he's written a ton about vertical leap, sprint speed, and how they can be meaningfully improved through better mobility and increasing strength.
http://www.higher-faster-sports.com/articles.html
It's all free, too. Can't beat the price.