The key to unlocking extra seats from the existing capacity is to prise back season tickets from those not using them personally. The Club has been toying with a season ticket amnesty for years, but hasn't yet had the balls to see it through, judging -- correctly I think -- that it would upset a lot of people.
Rather than continue with the current season ticket product, which is essentially a right to one seat that people are free to pass on as they please, let's abolish that completely.
Instead, I'd propose three brand new products, making clear that they are all for personal use only:
Season Ticket Gold - Package of all 19 games
Season Ticket Silver - Package of 13 games
Season Ticket Bronze - Package of 6 games
In general, the holder gets to pick which games they want at the start of the season, but with provision to do swaps later where there's availability.
Existing season tickets holders would get first dibs on converting their existing seat, followed by those on the waiting list.
The options available for the specific seat would be:
Convert to a Gold in the same name
Convert to a Silver in the same name
Convert to a Bronze in the same name
Convert to a Silver + Bronze, with one in the same name and the other in a new name
Convert to two Bronze packages, with one in the same name and the other in a new name
Convert to three Bronze packages, with one in the same name and the other two in new names
There would still be the option to return any given match ticket through the official exchange. You could also allow transfers from one member of the season ticket group to another.
If the clear understanding were that anyone caught passing on their new season ticket to someone else to use would lose it, then I think a lot of existing holders would opt for a combination of packages that amounted to less than the full 19 games. I think some people might quite like being able to pick just 13 or 6 games to see and not have to commit to them all. This could free up a lot of seats for members' sales, youth sales, etc.
Obviously, the details above are flexible. There's no reason why you can't have different numbers of games in a package, or splits between more people. There's also an argument to limit the number of the old Cat A games that can be in a single Silver or Bronze package so that people can't just cherry pick the best 6 games.