Yeah, Regionals will be bigger, but I don't really anticipate them being much bigger than the ~500 players that Fort Wayne/St Louis got last year. Those Regionals already attracted players from all around the country, and while they might bring in more players due to being completely exclusive on the weekends that they take place, I don't think they'll double in size from last year.
The Regionals like Virginia, Arizona, and California that had between 230-300 players will probably be the ones that double in size. And yes, it will be much harder to do well at them, but the best players will still walk away with some amount of points in at least half of the Regionals that they go to. Combine that with 120+200 point allowances from local events (assume a Worlds caliber player will always earn 70% of those points, 214 CP), and it's still relatively easy to achieve 300 points.
I really don't forsee getting 300 points to be strictly harder than last year (in fact, slightly easier, provided you attend about as many Regionals as you did Regionals+States). Combined with the fact that I don't feel that a 500+ person Worlds is really all that exclusive, I think 350 or 400 CP are better thresholds.
To the point that LCs aren't soft, that's true in many places. But if you have enough in your area (assume 3 per month, 9 per quarter, only the best 2 count), it's pretty easy to take at least 1 win and 1 top 4. LCs are usually best of 1, so there's a ton of variance. The best players won't do well in every event due to bad pairings, bad deck matchups or bad draws, and they won't even attend every event, especially if they get their 2 finishes early in the quarter.
And there are also plenty of areas where there are very few Worlds caliber players, allowing some weaker players to get "easy" wins. I attended at least 2 LCs last year where I was the only player in the room to have ever achieved a World invitation, or even top cut a State or Regional event. These areas are the ones where I especially fear 300 CP as being far too low to determine who should compete in the World Championships. Being the best in a mediocre area and never having to travel outside of it does not equate to being among the best in the World.
LCs are also always very competitive very early in the season and very late in the season. Players are eager to get the first points and also to close out an invite. In the middle months of prior seasons, we started to see declining attendance and I imagine we will in the middle month of the new quarters as well.