My principle problem with a diagonal lashing, and most other normal lashings, is that until tying the frappings, there is basically no use of mechanical advantage, witch usually isn't much of a problem. However, I spend a good deal of time teaching these lashings to very young boy scouts who tend to have little understanding of how to keep tension in the rope while tying the knot. This leads to very loose lashings that do not hold, and i would ideally like to find some lashings that are easier have new scouts tie given that they struggle to exert even half of the tension required for a truly secure lashing.
Also, despite us already having enough lashings to do most things, the number of lashings that i have seen formally described is stunningly few. The boy scout handbook describes 6 lashings, and abok isn't much better. I know there are many more ways in witch lashings may be used, but i feel we lack the proper array of lashings to do this. The majority of the lashings i have ever seen are only good for lashing two poles together, and although that is the most common use of lashings, there are many other knots that work to attach 3 or more poles. Currently, the only formalized 3 pole lashing i have seen is a tripod lashing, and that has a very limited use.
For example, at our last campout, being the most avid knot tier in our troop, i was designated to design a means of lashing together 4 catapults, using scrap boards. In this case, i feel that there are a number of special lashings that could be specialized to a flat board opposed to round poles, and that would be stronger or more decorative, but i made due with basic lashing for the most part. However, at the core of the design that we determined to use there was a place where 3 boards had to be attached to form the top of an A frame with a board running down the middle of it. Seeing how these were all flat boards that had to be fit against each other, a tripod lashing was insufficient, as it separated the boards from each other. The knot that i eventually settled on was a set of two sets of stitches like a tripod lashing, the one on top having the poles in revers order compared to the one on bottom, that was then secured with frappings around the sides of the middle board to hold the lashing tight. The resulting lashing was especially strong, particularly between nice flat boards, and actually less sensitive to how tight the ropes were than most lashings.
What i draw from this experience is that these types of unorthodox lashings should be organized somewhere so that we can have a more comprehensive list of lashings that is more useful than what is already out there to the common user. (also, from a theoretical standpoint, the set of barely 10 formal lashings we have right now seems incredible incomplete, considering how many things we could want a lashing to do)