I have always used TNF Paramount convertible pants. However, they are a bit heavy and I can't stand that the shorts come down to the tops of the knees. When I go to vent the legs, the opening keeps catching on my knees as I walk. They also have overly deep pockets and a few other quirks.

I was thinking of buying the REI Sahara pants with the legs that zip up the sides so you don't have to sit and take your shoes off to remove the lower portion. However, the reviews are terrible.

So, I was thinking: what if you had a nice, durable pair of quick dry shorts and simply added a lower leg portion using lightweight fabric? I could sew a lightweight zipper at the bottom of the shorts, and then use a velcro closure down the sides of the legs. The lower portion of the pants would be easily removable, able to be vented effectively, and would be lighter than standard pants and the leg portion would pack down smaller.

What do you think about this idea?

I'm sure that would work.

It would also probably be just as easy to buy a pair of pants you like, and add the zips where you want them to be.

I wondered about that.

I'm nervous about modifying the North Face pants...I would have to add the zips not at the seams because the top zip ends off to the side. However, velcro could still be an option.

However, I was thinking, if I make my own leg extenders, not only would I be able to save a little bit of weight and add functionality, I could also make them gaitors so that they attach to my shoe. Then I wouldn't have need for that extra item.

If all you want is 'calf zips' to remove the lowers without taking off boots, I'd imagine anyone with decent sewing skills could whip that up. I have 2 pair of Redington nylon outdoor pants that are this way. After a year of wishing I had, I plan on having this done before summer gets in full swing and I wish I had calf zips.

You mention that the TNF pants 'leg zips' that make them shorts are too long, so any modding of those is rather pointless, IMO. I still think that buying a pair of non-zipoff pants for cheap and adding zips to make them shorts, and zips to open the calves is a 'cleaner' option. I do recognize that I'm really OCD about this kind of stuff, adding legs to shorts is likely to look homemade and the fabrics wont match.

As for the weight... Clearly, that's a consideration, else why would you frequent a site for UL backpacking, but all four of my usual pants for hiking weigh between 10 and 11.5oz, sewing legs to shorts may net you a pair of pants that save you 2oz over commercially produced ones.

I think the gaiter idea is neat, but buying any pair of pants in an inseam longer than you wear would allow for this mod to be added to the cuffs. But then, the question becomes: What happens to your gaiters when you zip off the legs to be shorts?

I'm hoping this doesn't come off as argumentative, just furthering the conversation.

I thought about that also. I think you'd fold or roll the tops down?

Not yet sure.

What good, durable pants would you start with?

I'm thinking that if you rolled the leg potion of your pants down to make gaiters, you'd either have to hold them up around your calves with an elastic loop, or allow them to gather around your ankle, it would require something added to make them work properly.
This would be especially prevalent with the lightweight fabrics desirable in hiking pants.

If I were in your shoes, I'd start by shopping for zip-offs that fit well, and had an appropriate shorts-mode inseam, and add calf zips if needed after the fact. It's not hard to cut off belt loops, cargo pockets and flaps from existing clothing to trim disliked features and weight. Fit and length are the two hardest parameters to match.
I'd start there because zip-offs are essentially the same price as plain pants, and the hardest part has been done for you.
If that failed, I personally would look at something like the Mtn Hardware Mesa pants. I had a pair a few years ago and they were light and quite durable. They met their fate in an unfortunate incident that involved me, my motorcycle and an oncoming driver taking his half out of the middle, but I walked away relatively unscathed, considering I was wearing hiking pants in a slide/roll down a dirt road.

This is fun to discuss, and think outside the box, but light zipoffs and dirty-girl gaiters seem like the most effecient combo possible.

For comparison, here's what I'm wearing:
http://www.sierratradingpost.com/redington-versi-pants-convertible-for-men~p~48958/?filterString=s~redington%2F&colorFamily=33
They weigh 10.5oz, and the shorts come to about 3/4" above my knee when standing. I have about a 31" inseam.
If you were super intent on lightening up your clothes, removing the mesh brief, cargo pockets(all external pocket) and belt loops could turn these into about what I think you had in mind in the OP.

Some good ideas.

I have a pair of MLD gaiters now...
http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=37&products_id=114

But, I have to take my shoes off to put them on.

I was thinking a lightweight drawstring with a cord lock sewn into the pants could hold the gaiters up.

I don't know if it's a worthwhile idea or not.

I often wear the gaiters even when not "needed" simply to keep debris out of shoes and because the insides at the bottom of the pants were getting muddy and wearing and the gaiters fixed this.

Basically it would be a lightweight knee high gaiter that would zip onto your shorts and be removable without removing your shoes. Then if you wanted low calf gaiters, just fold the material down and cinch tight.

Sounds plausible.
It wouldn't be my favorite solution, but mostly because im really particular about fabrics being matchy-matchy.
If you got some lightweight material, and ran a zipper from ankle to the leg-zips, and put some 1/16 shock cork and a micro cord lock around it, it'd probably work alright.

I'd get some cheap fabric remnants for proof-of-concept before I spent money on good fabric and zippers and such.

I'd be interested in seeing how it comes out.