With the exhaustive lineup of Pokémon we already have, they could certainly just sprinkle a handful of new Pokémon in each region they release (10-20). They won't need to make a new game engine, won't need to make new inventory systems, menu systems, only minor additions to sound track and sound effects (if any), no new battle system, or a whole lot of things they typically start from the ground up with each new game release. What they would need are programmers, story boarders, and design artists for the new Pokémon, buildings, NPCs and the region itself. Since a large portion of the game is already done, I'd say $29.99 USD would be a fair price for a new region. That's $10 less then a 3DS game and $30 less then a whole new Switch game.
Ultimately, I don't think they'll do it. Nor am I sure I would want them to. I just said I've given it some thought. Would it be interesting? Sure. I wouldn't mind an open-world Pokémon game where I play a persistent character and can fight the gyms in any order I want (assuming they scale and use different teams based on how many other gyms I've defeated before them), but realistically I know I shouldn't get my hopes up for something like this. I guess the part that appeals the most would be that I am the Pokémon master and not the character in the game who changes each region, making my lovingly collected Pokémon split across various owners. I want a persistent trainer ID that I keep forever. That's really the only thing I want. If I could import my trainer from game to game that would be cool too.