Making the GDP’s possible range of values larger should be very doable with no new mechanics. You just scale down all the effects affecting the GDP by some factor.
One could probably “spoof” some kind of a GDP value by multiplying the starting GDP by a factor in some way based on the GDP statistic. But is that an oversimplification? I don’t know; I lack the economical knowledge for that.
But I can point out that there’s been some advocacy for renaming the GDP into GDP growth and piping that to an accumulator for GDP: Proper GDP simulation?
However, accumulators (besides debt and deficit) at least in the base game is against Cliff’s current stance:
A shame. I’d like to see a (probably thinner additional) horizontal bar of some meaningful numerical data about my country, and those figures being able to affect the magnitude of some other stuff. Some ideas for such statistics are discussed here: On accumulators
Even though I did also hear the claims about this game being an incremental upgrade to Democracy 3, I did see some potential in the developer having more interest in scaling the engine’s capabilities up when I noticed there’s now a disposable income screen, and the overton window also piqued my curiosity. But I’ve come to think our visions differ at least as far as the base game goes. He did talk about possible future modding support so there’s some hope in that regard.