More recurring characters (reducing the size of the main cast by one or two if needed). Recurring characters are great. You can put them through the ringer and take all sorts of risks with them that you can't do with the full-timers. DS9, of course, demonstrated this in spades, as did BSG.
"But wait", you say, "didn't they kind of try that in seasons one and two, what with Culluh, Seska, Jonas, and such?"
Yes, they did. Two problems with their implementation, though. One was that the plot was kind of a dud. Two was that, well, OK, it is probably too much to hope that all your recurring characters be the level of Mark Alaimo, Andrew Robinson, and Louise Fletcher. But a little more than what we got on Voyager was needed.
"But wait," you say again, "this is a show about moving forward, travelling toward home (which was part of the problem in seasons one and two), so how can you do recurring characters in that?"
Well, recurring characters can simply be permanent crew members that you don't see every week. But more broadly speaking, I would have made the show a little more like the template of the seventh season episode, "Void", in which Voyager is out there forming alliances, even making a mini mobile Federation (and making a few enemies in the process). And unlike "Void", I would have had these relationships lasting beyond one episode. And that means room for recurring characters.