If you're building your own twine sensors or need to replace the cable that came with the twine, you might find them at a TJ-Maxx store. I was in the store looking at the iPhone accessories on a rack near the checkouts. I found a twin-pack of 3.5mm audio cables with a male TRRS plug at each end. (They had several)
The cables are 5 feet long, and one is black and the other is white. The brand is iSound (www.isound.net) and the package of 2 was $7.99. At the iSound web site they are $9.99. Look for model "ISOUND-1609".
Twine "IS" the sensor interface. The idea is you place Twine where you need to sense things. To have Twine connect to yet another wireless device located in another location seems to defeat the concept. If you want extra sensors, get extra Twine's, that's the concept.
As for USB being an interface standard, it's all good and well to say that, but USB has specific standards they'd need to adhere to. 5v power is one of them, which Twine does not produce. Which USB would be used? (A, B Mini, Micro, USB 1, 2, 3). And it's not wise to have a USB plug on something that doesn't conform to USB protocol. A USB device probably needs to be registered somehow, and needs extra certification... all this means extra cost. The audio style connectors are readily available, simple, don't have a set standard of use or connection pinouts, are small and easy to use, so I think they were a wise choice.
If you REALLY wanted a wireless sensor (even though it seems wasteful), they'd be better off designing their own wireless transmitter/receiver pair. It would only have to transmit when there's a change in status of the sensor, and they wouldn't have to conform to Bluetooth protocols. It'd likely be cheaper too.
All these things that you'd like for Twine, increase the cost. Bluetooth is a hefty investment! It draws even more power, something they're working hard to get away from.
For example, I want to have a high level alarm for my house sump and a low temperature alarm from a point somewhere near the thermostat, for when I leave my house for more than a few days in the winter. distance involved, accounting for corners and bypassing of objects, 30 feet. The present configuration, does not allow me to do this, as shipped, or with any available TRRS cable However, in fact one can extend the moisture sensor electrodes indefinitely by clipping a pair of copper wires to them, stapling the other end to a stick, and fastening that into the sump with about an inch of the wires exposed at an appropriate level.
But, if Twine is to go anywhere, the off-Twine sensor model needs to change. It cannot remain only "on/off". It cannot be restricted to only one external sensor. It cannot use cables that are expensive and have limited availability. (Implicit, it cannot force only one external sensor of a particular type, and multiple sensors of the same type must be distinguishable by the Twine. If this is not clear, think door contacts; you must be able to have more than one, and the Twine needs to be able to tell which one opened.)
If Supermechanical continues with a wired sensor model, I would suggest USB as a connecting standard, with perhaps a 4-port hub on the Twine, and support for external hubs. Supermechanical would define the communications protocol, but outsource sensor development.
My preference would be to get away from wired sensors all together, after all, isn't the Twine idea to be free of wires?
Hence, my suggestion for Bluetooth sensor connection. The Bluetooth spec meets the Twine purpose very nicely, with connection distances ideal for household use.
I am of course talking Twine 2.0; we are probably at present at about Twine 0.3.
My thinking would be to go to a 1 (not so cheap) Twine, but lots of various "cheap" sensors, with a simple programming interface that is extensible to something like "Twine Basic",
I bought in on another Kickstarter project called SmartThings they are doing what you are talking about here, a base unit that talks to multiple wireless sensors. I always thought of Twine as a multipurpose device but only one purpose at a time. Twine is going to be a lot more fun as a development platform where ST is a finished consumer product.
Look up Ninja Blocks for more functionality and programmability. But be aware that with more functionality, comes more power usage, and that means batteries aren't an option. Which is another reason they can't make TWINE much more powerful (processing wise), batteries won't cut it.
If you want a boat, you don't buy a car, if you want a car, you don't buy a boat. If you want multi IO, you don't buy a TWINE.
TWINE will probably always be a hobbyist/geeky type gadget, that will not change the world with it's current form at all. However it is another gadget that will make Supermechanical and/or other manufacturers develop more suitable items for the masses. By "masses" I mean the types of people who watch Big Bang Theory, AND understand the innuendo's. It's not a consumer product, so it'll never have the backing of it.
Twine and Knut (http://www.amperic.com/) seem similar, then there is SmartThings and also Wireless Sensor Tags http://www.mytaglist.com/ . I'm betting there will be dozens more arriving in the next year, some from well known companies and others sprouting up in garages around the country.
It's an exciting time to be a part of at least one of these initiatives. I'm going to be involved as much as I can be with several of these - right now I'm using Twine and Wireless Sensor Tags. Def looking forward to seeing what SmartThings brings to the table when they launch in the next few months.
Let's hope the SuperMech guys are thinking ahead!