I know it's a pretty damn reliable system as is, but a little extra won't hurt.
The only thing I can think of would be the LMT Carrier.
Anyone else have any suggestions?

btw, I have a 16" Bushy with a CAR stock, spring & buffer.

My rifle is the same,I added a heavy buffer and I added a lesbaer chromed extractor and ejector so its harder to gunk up those parts.I went to the extractor cause I use wolf for blasting ammo and I didnt want premature wear from the steel case even though the mild steel case is way softer than the ordnance grade steel a fire arm uses.I did the ejector cause I figure the chrome is slippery and carbon does not adhere to it so I figured it was less likely to gum up incase I had to shoot alot and not be able to clean it.I have gone 2000 rounds without a cleaning before changing these parts and 6000 rounds later they are going great althogh Im due for a 2 or 3000 round test to realy see if they can take it the way the original parts did and those parts are still good.The LMT carrier I would love to get if it was not so expensive,I am waiting to see if the design is adopted by the military then if it is every one will probably make it and the price will drop.And now with the adoption of an even heavier M4 barrel seems us HBAR lovers were onto something.The new barrel is 10oz heavier than the original M4 barrel and 6 oz lighter than a regular HBAR because its still machined to take the 203.If the military ever switches to mounting the M203 say quick release on the bottom of the RAS rail they would probably go full HBAR seeing as how its more reliable under Full-Auto or rapid fire conditions,No barrel whip and with all the gadgets used today and extra pound in weight is realy not a factor any more if it were they wouldnt have gone heavier than the original spec to begin with.

I do think that all evidence says that a 20" rifle is more reliable.

The M4 heavy barrel was not "adopted" and is not the standard M4 contour.  A few people have them and there is no plan to replace current M4 barrels with HBARs.

More reliability?  Stronger extractor spring, M16 carrier, H buffer and extended feed ramps is all I would do in your case.  Other than that, if it aint broke dont fix it.

Some items like Dfenders, McFarland gas rings, Fat boy gas tubes, etc can actually cause a problem if you dont have a problem to begin with in certain instances.

The most important thing you can do is perform proper and regular maintenance.