I imagine a future-proof tablet would be modular and easy for consumers to modify, like PCs. Modularity for PCs is possible in-part because of standard form factors. But this is something that hasnt been fully embraced by mobile device manufacturers (to my knowledge).
I would love to have a tablet where I could easily add memory, processor, screen, etc. So instead of buying a new one every year, I can just upgrade and extend the hardware myself and still run the latest software.
The problem is that having user-serviceable parts significantly adds to the bulk of a device, and it's pretty clear that's not an appealing tradeoff to most users and manufacturers.
E.g.: just to have a removable battery, you have to put a protective case on the battery as well as an internal "case" to receive the battery inside the device (generally, another layer of plastic. Even on laptops that is becoming prohibitively bulks; on tablets, it's a non-starter (see the recent MacBook Pro redesign). You just can't pack the parts together tight enough and still have them be user-replaceable.
I love the idea of a modular mobile device as well, but it's pretty clear things are headed in the other direction: thinner, sleeker, longer-battery-life products sell better than bulky but more customizable products.
I would love to have a tablet where I could easily add memory, processor, screen, etc. So instead of buying a new one every year, I can just upgrade and extend the hardware myself and still run the latest software.