NOTHING. You don't need to replace any parts to ensure long life.
As stated, regular care is all it needs.
Avoid the chrome plated bolts, carriers, titanium firing pins, adjustable gas systems and all that crap. Solutions looking for problems that cause more problems.
Your Bushie will last a lifetime. Sure, the barrel will need replacement at 20,000 plus rounds, and you will replace some cam pins and such, but that should be about it.
What makes you think you NEED better parts? Sometimes we spend a lot of time and money buying gadgets that are supposed to be "better" that really don't help at all, and often hurt.
Spare parts are another issue, and are great to have. Bushmaster sells some nice kits with most everything you'd want.
Nothing wrong with a spare gas tube, but don't forget to get a few spare retaining pins, easy to lose.
I just replaced a barrel and had to clamp the gas tube with vise grips and pop it with a hammer to remove it. It's TOUGH! I was planning on destroying it if needed, but I didn't even dent it.