Basically, is there a slice of the pie in trading much faster than humans, but much slower than HST?
It's an academic exercise, but one I've been toying with.
One thing to keep in mind is that the higher your trading frequency, the smaller the price moves you can hope to capture, which limits how much capital you can deploy in your models. This is why HFT models are usually small in size but have high Sharpe ratios. As you reduce your trading frequency, you can expect to capture larger price movements and deploy more capital but you’ll also be exposed to more of the vicissitudes of the general market, so your Sharpe ratio will decline. Market participants usually carve themselves a happy spot on this frequency spectrum and stay there. I don’t know of any firm who is successful at every spot.
All the inputs to their pricing change rapidly, so their order prices must change quickly as well, but they can end up carrying risk for long periods of time. The Australian regulator looked at HFT activity in their markets, mind you probably less sophisticated than US stocks, and found the average holding period was 42 minutes: http://tabbforum.com/opinions/hft-concerns-are-overstated
Not HFT by any definition (unless your definition of HFT is on the order of minutes), but purely algorithmic and data-driven. You can backtest your algorithms with up to 13 years of historical data, live trade your algorithm with paper money, and even link your algorithm to a broker and trade your algorithms with real money.
It's the single most disruptive financial service I know of, and I've had tremendous success with it, even without linking a brokerage account (just checking it daily for trade signals in my super-low-frequency algorithm on paper money).
For the "faster than a human, slower than HST" niche all I can think of is the traders who automatically react to press releases, twitter and the like. If you're getting data that no-one else has and it affects the price, it doesn't matter whether you're as fast as HST. (Of course if you get competitors who are doing the same thing you still have to be faster than them).
Beyond that most stuff happens at human speed - which doesn't mean you can't do market analysis with algorithms, competing with humans. But speed is always going to be a factor - even if we're talking about e.g. a multi-month analyst investigation that figures out that company X is really a massive fraud (there are trading firms who make their business figuring this stuff out), that still becomes worthless if your competitor finishes their investigation a day earlier than yours.