Replacement windows have such a positive effect on your ability to conserve energy and remain comfortable in your home. But if you really want to make the windows as close to perfect as possible, you can add some accessories that will increase your comfort and privacy while enhancing safety. These are easy to find and install; you just have to find the right styles for you.
Weep Hole Screens or Plugs
Window frames have weep holes that allow accumulated condensation to drain out of the window tracks instead of overflowing into the room. While you shouldn't have major issues with condensation on new windows, you can still have some if the window is located in an area with poor in-room ventilation or if your preferred thermostat settings are still relatively cool in winter. However, the weep holes can let bugs crawl in, and newer insulated windows have fairly big weep holes.
Get plugs for the weep holes that block them off with screens, or stuff steel wool into the holes. Water can still drain out through those, but bugs can't crawl in as easily. These options are both inexpensive, easy to order online, and easy to install.
New windows come with great locks, but for slider windows, it's always nice to have some extra protection. Pin locks or thumb locks placed on the track right next to the sliding pane can prevent people from shoving the pane over in an attempt to get in. Pin locks are very strong but require drilling into the frame; thumb locks are temporary brackets that cost only a couple of dollars for a set, if that, and they are movable. With thumb locks, too, you can leave the window open a tad for fresh air while still locking it down, though you should close and lock the window completely if you're about to go to sleep or leave the house.
In addition to the blinds or full curtains you'll have on the window, invest in curtain liners - those sheer, gauzy or lacy curtains that allow light in but block clear views into the house. These are really nice on windows that face the street or that face a neighbor's house. Instead of opening the blinds or curtains and wondering if anyone can see what you're doing, place the liners on the window, keep them closed, and open the blinds or full curtains. Note that people can still see inside if they come right up to the window, but if they stay on the street it will be harder for them to see what's in the room.
Windows should give you light and air, and make your home a pleasant place to be. Talk to the window contractors who are installing the new windows and see whether they might have some of these accessories that you can look at.