Because the cards are random (which is possibly the downside to the game overall, but I like it well enough) I found it hard to isolate yourself to 'be solitaire' in the two player game. The trick seems to be not to give up too many sweet spots for the other player to pounce on.
Just adding another start tile or two might make things more interesting.
The additional start tiles might be interesting. Even with two players, the benefits of keeping your developments and farmers near existing settlements (in additional scoring) and the inability to keep all your cards for one area, or keep your opponent out, meant that things seemed nice and interestingly clustered around two big settlements in the game.
More start tiles might allow people to break that up into more starting settlements, diffusing the action even more?