The Talon grips look good, but it was cheaper to put a short piece of "skater tape" (aka 3M stair safety tape, from Lowe's or Home Depot) across the front strap for extra grippiness. (The first time I did this, I just wandered into a Skateboard shop and asked them for a small piece of tape, which they gave me for free! A store-bought roll of 3M stair safety tape should last a lifetime).
I like the Pearce pinky extension at the bottom of the seven round magazine. These cost less than $10.
I also agree with the bulk of the advice in this thread, though: shoot it a bit before you see what else you might want to add. And buy ammo first!
I am adding a Sintercore end plate with "wings" (called the "Deadfall") to my new Shield 9mm to make it easier to operate the slide; it is on backorder and due in June. That cost $29.00.
(Update: the introduction of this item was cancelled on May 30th, based on "lower than anticipated interest and unexpected engineering challenges." A company named Wright Shooting also makes something called the TacRack which does the same thing, and is available now, for about the same price. I'd provisionally recommend the Tac Rack, which is fairly large, if you have trouble racking the Shield's slide all the way back.
Then there is the issue of the Apex trigger. That might be a good thing to add later, but that depends of whether you like your Shield the way it is. I don't think the Apex trigger is "required" to make the Shield a good gun; it is just a matter of preference. But you do need to shoot it a while to see if adding the Apex trigger is a good idea
for you.