I like the idea - especially given how I've never been able to complete the pok?dex apart from the golden period between the release of HgSs and the end of the GTS in Gen IV - but I feel like in practicality it wouldn't work unless we're making a game the length of something like Assassin's Creed on a console. In your average sized pok?mon game wee'd go from arguably having far too little variety in many games, to having like 8 different species of pok?mon in the first route alone.
I disagree - I think it'd be quite possible to have a complete Pokedex in your average-sized Pokemon game. I mean, 802 sounds like a big number, but when you remember that there are 61 Legendary Pokemon, a lot of Pokemon that evolve via special methods (i.e. evolutionary stones, trading, friendship, etc.) that you wouldn't find in the wild anywhere, special Pokemon (fossils, and other one-offs like Type: Null) and second/third stage Pokemon you also wouldn't find in the wild, it cuts the number down pretty dramatically...I'd say the number of Pokemon you'd need to include as catchable in the wild would be closer to 500. I haven't figured it out precisely, but there are an awful lot you wouldn't have to have available to still be able to obtain them.
I don't think it that difficult, if the designers pull their fingers out of their collective arses and stop doubling up constantly on what is available along what route, and - more importantly - make use of all the game functions. Let's say there are an average of 5 Pokemon per route - 4 common, and 1 rare. So, how could they add to that without overpopulating them?
- Day and Night cycles
- Seasons
- Weather conditions
- Pokeradar/Dexnav
- Mixed terrain (i.e. grass patches along water routes, ponds along grassy routes, etc)
- QR codes
- Rustling grass
- Swarms
- Horde battles
- Days of the week (like how Drifloon was only available on Fridays)
- Headbutting trees, if there are any
- Version exclusives
These are just for routes, too. We also have landmarks like caves, man-made structures, Victory Road, etc. ORAS demonstrated that there is no issue with adding in a ton of Legendaries to the games, and asides from events like that there are also (sigh) roaming Legendaries that can be added. There have always been events where you can get one-off Pokemon like Type: Null, Riolu, Zorua, fossil Pokemon, etc.
The problem isn't lack of areas to put them, it's lack of variety in those areas, and Game Freak's stubborn stupidity in constantly cutting out features, or not utilising them to their full potential. Having the same Pokemon and it's evolutionary stage in every route in the game. Maybe the game would be a bigger undertaking, but the average-sized Pokemon game (in terms of places to explore, etc) wouldn't have to be, I don't think. Not with what we currently have, at least.
And I'm afraid the "it's more work for them" excuse just won't cut it, when the quantity of Pokemon available in titles, and the size of the regions, varies dramatically. There is no reason they couldn't do this. They just don't want to because it'd probably be seen as a "definitive" version of the game and people would have no reason to buy other versions after that. It's a reason that they don't do, but not a reason that they couldn't.