diversifying; but isn't that what mutual funds already do?
They diversify and reduce stock-specific risk by moving from individual stocks to many stocks, but you can diversify even further by selecting different fund types (e.g. large-cal, small-cap, fixed- income (bond) funds, international, etc.). Your target-date fund probably includes a few different types already, and will automatically reallocate to less risky investments as you get close to the target date.
I would look at the fees of different types of funds, and compare them to the historical returns of those funds. You can also use things like morningstar and other ratings as guides, but they are generally very large buckets and may not be much help distinguishing between individual funds.
So to answer the question, yes you can diversify further - and probably get better returns (and lower fees) that a target-date fund. The question is - is it worth your time and effort to do so? You're obviously comfortable investing for the long-term, so you might get some benefit by spending a little time looking for different funds to increase your diversification.
Note that ETFs don't really diversify any differently than mutual funds, they are just a different mechanism to invest in funds, and allow different trading strategies (trading during the day, derivatives, selling short, etc.).