New starters playing in old season is already causing issues with the player experience, and for people in the later seasons who started the game earlier the player count is dwindling with no way of replenishing the numbers.
I suggest that all new players start in the same season as everyone else, however everyone has a story journal where they can go back and complete the story content from previous seasons. In doing this everyone can receive a good selection of rewards enabling them to obtain champions and resources from all seasons and give them the ability to compete in the current season from the very beginning.