In all my years of trekking the back and high country, I have wished many times that such a modular system existed...and tried many times to invent it.

Unfortunately due to many factors that have to be taken into account such as (and not limited to) ecosystem, terrain, time of year, weather, distance, your physical condition, number of people in the hiking group etc. These factors then have to be examined closer, for example: What is the defintion of good weather, what is your definition of a day trip, what is your definition of a good meal at the end of day, what is your definition of adaquate gear, what is your definition of easy terrain as compared to others who may also be in your hiking group...and so on.

When you factor in all the above (and more), having a one size pack that fits all that can then be added to via a modular ssytem is not an easy undertaking.

That being said and as I mentioned above, I spent years wishing it was this easy however I long ago found that such a system does not exist that works for me. One critical thing I do not like about a clip on type of modular system is the weight distribution shifts as you add gear to the outside of the pack. This may not be critical if you are hiking along gently rolling land with a good trail, however it can mean the difference between life and death as you precariousy pick your way up a mountain goat trail you are following to get to the peak of an 9000' mountain.

What does work for me though is having a very broad and diverse base kit system to choose from that allows me to tailor each and every hiking or camping trip to it's specific need..and also allowing for some flexibility for other unknown factors that may arise later. This base kit is always in one backpack and can be easily transferred to any of the 7 other backpacks I own. These backpacks all have a specific purpose and range from small daytrip size to good weather / 1-2 night sizes to expedition size that will easily carry enough gear for week long remote trips. No matter what gear I take, it will fit in the back pack of choice and I don't have to worry that my cookware which was attached to the outside of the pack, fell off 7 miles and 4000' behind and below us...yes I have seen this happen to someone.

If you really are intent in carrying a modular system, there are very few civilian pack designs that would work and you should really look into a military based MOLLE system.