Nintendo can do whatever the hell they want, but there aren't massive changes to battery tech, and the physics of heat haven't changed either.
If Nintendo wants the same battery life in the same size device, they need the same power draw.
The general assumption is that the launch Switch's battery life is lower than Nintendo wanted, but was a consequence of an off-the-shelf chip built for laptops. That matches Nintendo's behavior at the time: they aggressively downclocked handheld and then had to backtrack very close to launch in order to get ports (including Zelda) working and they quickly shrunk the chip.
If the Switch's launch battery life wasn't good enough, and Nintendo is designing this chip from scratch (so unlikely to have more than a sub node improvement), Nintendo will either have to make the device bigger, or drop power draw.
Similar for heat. There is new tech for heat management, but that only addresses one half of the equation - how do you move heat fast enough to protect the chip. A vapour chamber is good at protecting the chip, but it still makes the heat still has to go somewhere. For a totally stationary console, put a big vent in. For a phone, heat up the backplate, and trust no one will run it at high speeds long enough to get uncomfortably hot. For a handheld that needs to support a docked experience, that you expect children to be using, your options become more limited.
If Nintendo gets good battery life and ergonomics with a higher power draw (and thus more performance), I'll be happy, but I think to keep ourselves sane, we really have to imagine the launch Switch's power draw as a ceiling, not a floor.