You may have missed the forest for the trees....Similarly, again, look to the pricing of mifi modems. Strip away their parts that wouldn't be needed (for example the outer case since the iPod's outer case would cover that). If one can buy a whole, stand-alone 3G data device for $100 retail (sometimes one sees $79 and even $49 retail), I would think Apple could build in the underlying tech for not much more than the implied costs involved in making whole mifi modems. Then, those that don't care could buy the "without 3G" iPod and those that might want it could buy the "with 3G" option... just like on the iPad.
On the iPad "with 3G" adds $130 to the price. If an Ipod Touch 5G was available with a "with 3G" option for those interested (and data options that matched the no-contract options of the iPad), I, for one, would opt for the extra $130 to include the 3G flexibility. I'm not saying it should be forced into EVERY iPod but in my own case, +$130 for a 3G option on a gen 5 would get my money in the next month or two; no 3G and the money most likely stays in my pocket. If I am not a one-man market, there's probably a nice little pocket of opportunity there for Apple... those that don't want the total cost of thousands for an iPhone but could use the occasional 3G access via an iPad-like "use it when you want" 3G plan on a tiny device that fits in one's pocket.
I'm hoping Apple is thinking "iCloud on everything" so they include a 3G option (hopefully choosing to make it available for less than +$130 to entice more people to have iCloud-anywhere access); but I bet the big selling point (probably unstated) would be the on-demand, "cheap" data-only plan hooked to a portable iDevice which seems like it would be perfect for all those "text anytime" teenagers with parents that don't want to pay the full voice+data toll that comes with an iPhone. In my own case, I'm not that teenager, but I do have an occasional need for 3G access (certainly far from enough to ever buy an iPhone). In the past, I've spent my money on other company's technology to buy mifi modems etc. If Apple would rather have that kind of money instead, they build that mifi right into the next iPod Touch.
There are an awful lot of teenagers hungry for an iDevice but not the AT&T or Verizon monthly toll. They want to text but not much voice, though AT&T & Verizon forces the voice plan cost to get the data plan. If the parents balk, the kid ends up with Android or less. If Android, they accumulate Android apps while they are teenage poor. When they can pay up for their own "real phone", they would be faced with a choice of iPhone 7 or 8 and re-buying a bunch of their favorite apps again or sticking to the platform where their app investment already works. Thus, even though this penny-pinching market might not seem ideal for the short-term, the availability of app platforms does make it a longer-term upside market. Apple should get it rather than giving it to Android or others. A few years from now, those poor teenagers are prime candidates to pay up for the next-next-next gen iPhone. Already being locked into iDevice apps would practically insure that they buy Apple in the future.
There are prepaid Android smartphone options for $25/$35 a month for full text and full data plans. That is the going rate for 3G iPad data plans alone. Penny pinching teens aren't going to shell out an extra $130 for a 3G iPod Touch ($130 being the added cost for 3G over a non-3G iPod Touch) and an extra $25/$35 a month just for 3G access for their media.
If they can afford the more expensive iPod w/3G and data plan and a cellphone, then they can afford an iPhone. (the cost is virtually identical) And they'd have the added benefit of carrying a single device, not two.
Considering how cell providers are capping data, it would be illogical to depend upon 3G as an integral part of any "iCloud anywhere" strategy.
Apple would need to do something radically different than what is currently available in order for the scenario you painted to be feasible.