There's no way an add-on that lets you play the latest and greatest games will be relatively cheap. Plus, no one will program software for it.
I think an external hard drive that has the minimal processing power necessary to unlock bonus features for a reasonable price is a much easier sell than buying an upgrade for your console. Plus it fits nicely into Nintendo's track record of including the bare minimum of storage in their consoles. The benefits to casual customers will be cheaper games. That's an easy sell.
It'd be no different from the PS4 to Neo situation, or 3DS to n3DS. All they'd need to get developers to do would be to program for the base model then add higher resolution and better texture settings for the module upgrades, with the option to also improve effects and frame rates and so forth if they wanted. However, it would have the advantage of being cheaper (though no, not 'cheap') than buying a whole new system.
Also, you're seriously underestimating how hard it is to sell anything other than better looks to the general consumer. Even extra storage is more confusing to most people, it's why we get ridiculous situations like people paying 100 quid for 16gbs extra internal storage on their phones instead of paying a tenner for a 32gb sd card, or Sony selling their 1tb PS4 to people whose 500gb version is full.
But stuff like 4K being better than Full HD which is better than just HD, that's something you can sell to people. They won't know what it actually means, but they know higher is 'better' and will spend money on it. Having a a full HD or 4K expansion pack you just slot in to make everything 'look better', people will get at least a rough idea what those things mean, even if they do more than just bump the resolution.
I do fully expect them to have fuck all internal storage though, but I can't see them pulling an Xbox 360 proprietary storage move with being able to expand it, both because there's no innovation in that, and even with a processing boost, it's not going to be a big enough or marketable enough gimmick to build the system around.