Now that I think of it, the chances of facing Red are slim to none. They'll probably end the game with the Elite 4.
There are other factors that I feel like could really make this more annoying than enjoyable.
-GSC has the absolute worst roster in the whole series. Cyndaquil/Totodile and the Spearow we could steal will probably be the only worthwhile Pokemon usable. If we can catch a Geodude, Gastly and Mareep then they're sorta decent. Everything else downright sucks, even Gyarados because its best moves are special. You might say "We had average Pokemon in R/B" but you have to remember pretty much all of our success was due to dumb luck vs stupid and glitchy A.I
-Which brings us to the next point. Battling is gonna be a huge pain the ass because the A.I actually sorta knows what to do. There's no more BS factors that could go in our favor.
-This one might be a positive to some but I see it as a negative. Besides Ice Cave, the region will be such a breeze to go through. There won't be any hurdles to worry about because Johto is so barren
-EXCEPT ledges. Lots and lots of ledges. Hope you like ledges.
-The level balance is terrible. By the time we reach Blackthorne city, Claire's team will be full of Pokemon at level mid 40's whereas typically we should be at 30's at that point (although we'll probably have one overlevelled Pokemon and bunch of lower level ones since its TPP but this is even worse). We can't even grind much because the wild Pokemon are all at low 20's. All.
-Moreover, going back to the roster, the wild pokemon offer pretty low EXP distribution on top of the lower levels. EXP won't rain at us here compared to R/B, it'll be a drought.
-Miltank
-Poliwrath
-Steelix
-Kingdra
-E4