What are you going to use the bag for? Not a trick question. Tournament co-angling? Just everyday fishing out of other guys boats? Bag for your own boat? Bank fishing?
If the 3600 bag is stuffed - the easiest/cheapest approach is to re-evaluate what you carry and leave some stuff at home. If you're bank fishing - a back pack might be a better option.
A back pack isn't a bad option is you're fishing co-angler. It is generally good to only make one trip from truck to boat. Since I don't know your situation, let me relate how I've used different bags over the years.
I have an 18 'Lowe, that has much less storage than you might expect, non of it lockable. I have a Cabelas Magnum bag that holds a dozen different 3700 size boxes. Basically this is my traveling crank bait collection. Lipless cranks - shallow/medium/deep divers, etc. I don't keep this stuff in the boat because non of the storage is lockable - so I keep it locked up in my fishing truck. My reasoning ( confirmed by my insurance agent ) if stuff gets stolen out of my boat - no coverage. If stuff gets stolen out of my fishing truck - full coverage. If this bag isn't in the boat or fishing truck it Is in its own storage bin in my fishing shed.
I have numerous smaller bags, that hold between 3 and 7 3700 size boxes. I use these bags for the various kits that I might need from time to time. One bag is for Neko rig stuff. Obviously I don't have 3 3700 boxes of Neko rig terminal tackle - just have one. The rest of the bag has baits and other stuff I might need when Neko fishing. (Own a good, wind proof butane lighter. Insert weights insert better into elaztech baits if they are heated pretty good prior to trying to insert them). I have another bag for Biffle Bug fishing. Hooks/weights & rigs fit into a 3600 & 3700 box. The rest of the room is different baits appropriate for Biffle Bug fishing. Along these same lines I have a bag for Jika Rig gear ( jika rig bag has 3 small boxes of home made jika rigs, rest of the bag is filled with baits that I like for jika rigs, i.e. brush hogs, larger craws, magnum trick worm, various lizards) a different bag for Ned Rig Stuff. There is a jig bag. This holds trailers & pork, also jig boxes. Jig bag holds 5 or 6 3700 boxes. I use 3600 boxes because I like being able to find stuff in the bag without unloading it each time. Jig boxes get swapped out depending on where I'm fishing. Might be 3/4 ounce skirted football jigs, might be just a box of football heads, maybe finesse jigs,
maybe 7/16 oz finesses jigs, maybe light shakey heads, maybe heavier shakey heads. Never know until I decide.
Check out Harbor Freight - occasionally their 15" canvas tool bags go on sale for $5 more or less. This lets me keep a strong assortment of baits - kinda sorted by type - more or less protected from the elements. I keep a bag for senkos & another for stick baits that I'm going to tx rig - a bag for bubba drop shot baits, a different one for 10" worms. 7" ribbon tail worms get their own bag and so do tubes & soft plastic minnow imitators ( flukes & such ). Still not through, Chompers get their own bag, paddle tail worms & smaller soft plastic swim baits (Those less then 5" long). These Harbor freight bags will hold 15 to 20 bags of soft plastic easily, 30 if you stuff them in. Believe it or don't, but these bags don't even start to hold all the soft plastics I've collected over the years. All of the overflow is sorted into plastic shoe boxes, sorted by brand - size - etc. All of the canvas bags fit into an oversize beach bag that I came across on sale at Academy.
I'm pretty paranoid about stuff getting robbed, (even though it has only happened once ) so each bag is labeled both by what is in the bag and with my name & address. Name - address - phone number are also concealed inside the bags in different places, just in case they do get stolen, marked up, but the crooks don't check every nook & cranny.
When I was fishing as a co-angler in tournaments, I had a different system. Mostly fished Ozark lakes, which while each one has its own personality, they each fish similar, given the season. So I had a spring bag, a summer bag and a fall bag. Of course, there was some overlap and I was ok with that. I considered putting together separate bags together for each lake for each season, but never got around to it as my wallet didn't allow for that at that time.
Hope these tackle sorting suggestions help. A general rule that has helped quite a bit for me, is that in general it is better to have a larger bag half or 3/4 fun than it is to have a smaller bag stuffed to the gills. If you've had issues remembering what is in which bag - believe me, I've been there. For me - labeling is key. Colored duct tape & permanent ink sharpie markers work good and if you want to change the contents, just rip off the tape & re-label.
Do this a few times and your bags will start to look ratty looking, but I'm ok with that. The only thing I'm not ok with is zippers that don't work. Bags with bad zippers are more trouble than they are worth - pitch them and eat the losses. I haven't found a tailor yet who will fool with replacing zippers on tackle bags yet.