If i were you, the next thing i would buy would probably be a vertical foregrip*, but the person who said good boots are a very good investment is spot on imo. Knee pads are also very useful, less so but not to be ignored are elbow pads. How happy are you with your eyepro? I have a lot of options, most of which are good for specific purposes, none of which are perfect for any game, but having plenty of options allows me to choose the best couple for any given day.
*Assuming your gun can accept one.
And +1 to spare battery also.
A hydration bladder and backpack/pouch is great also - being able to sip without having to mess with a canteen is quicker and less involved, so you tend to stay more hydrated, which improves performance. I would say that 3L is too much - really a 1L bladder is enough, but for some reason finding pouches for them or 1.5L isn't easy and they tend to be more expensive, so a 2L and just don't fill it so much is probably the best way to go. Don't imagine that you need to spend loads for a name like Camelbak, etc - the cheap Chinese ones are just as good and even if you get one with a simple bite valve, if you want the extra functionality of a removeable valve plus tap (so that you can decant water from your bladder into a cup say), you can get them for about £8 as an add on.
Maybe consider things like gel insoles for your boots and/or thousand mile socks (although i prefer Marks & Spencer thin 'gents' ankle socks against my skin, with a pair of thicker walking socks as the outer layer - which gives the same double layer as thousand mile socks for blister protection); maybe Sealskins socks if your boots are comfy but not completely waterproof.
Do you have gloves which fulfil your needs? Can you feel everything you need to through the fingertips? Are they tough enough to protect your hands when crawling through bushes etc? How warm are they? I have more glove options than i have boots...
If I were you i would not buy a pistol yet - they are a laugh but very little use, unless CQB is your preferred venue. Better to practice with your primary and get good at quick mag changes, transitions to your off hand side (for the times when you need to lean around cover to get at a target), plus the all important muzzle awareness (whenever your finger is on the trigger, you should be looking through/over the sights and keep from pointing it at friendlies) and accuracy.