Why does it have to be a “season” ticket? Why not sell a ticket that is eligible for the next 20 league home games. Or you could sell tickets for all different amounts of home league games - 15, 20, 25 and so on. The higher the matches the lower the per game cost. For example £19 a game at 15 games, £18 a game at 20 games.
Then it can be used for this season (if games are eventually played), next season (whenever that may be) and even season after. People can be comfortable putting money into the club knowing that at some point they are going to get their money’s worth and it won’t be affected by cancelled games, closed door games or league reconstruction.
Current season ticket holders get discount/extra games since they have not had all of the home games this season.