An open fixture after Round 17 would be the best and fairest way to do it with 18 teams IMO. Given commercial appeal is an important priority to go along with the integrity of the competition, this is what I came up with for a fixture a couple of months ago, which I think is fairer and more interesting than the current set up.
Minor Round (Rounds 1-17)
17 Rounds – each team plays each other once with 8 home, 8 away and 1 neutral game. Fixtures reversed following year.
WEEK BREAK AFTER ROUND 17
Qualifying Round (Rounds 18-22)
5 Rounds – 3 groups of 6 teams based on positions at end of minor round. Play each team in your group once (top 3 of each group get 3 home and 2 away matches, bottom 3 get 2 home and 3 away matches).
GROUP A (Positions 1 – 6)
Top 5 start in those positions in Finals. Bottom goes in as Seed 1 to Round 23 sudden death match.
GROUP B (Positions 7 – 12)
Top 4 go in as seed 2, 3, 4 and 5 in Round 23 sudden death match. Bottom 2 eliminated.
GROUP C (Positions 13 – 18)
Top goes in to seed 6 in Round 23 sudden death match. Bottom 5 eliminated.
Round 23 Sudden Death Matches
Seed 1 (Bottom Group A) vs Seed 6 (Top Group C)
Seed 2 (Top Group B) vs Seed 5 (4th Group B)
Seed 3 (2nd Group B) vs Seed 4 (3rd Group B)
3 Winners fill positions 6, 7 and 8 in Finals based on seed.
FINALS
Top 8 with same system used as now.