A possible improvement to Single Non-Transferable Vote (SNTV) could be adding a possible Runoff round, so that the effects of spoilers and overly popular candidates may be reduced.
Single Non-Transferable + Runoff Vote (SNTRV):
1. Each voter casts one vote for one candidate in a multi-candidate race for N offices.
2. If the N candidates with the most votes have a sum vote total that is more than the Winning Threshold, then the offices are filled by the N candidates with the most votes.
[Winning Threshold = Floor of ([Total valid votes] * N / (N + 1) +1) = "Inverse Droop Quota"]
[Example: Total valid votes = 100, N = 4, Winning Threshold = Floor of (100 * 4 / (4 + 1) +1) = 81]
3. If the N candidates with the most votes have a sum vote total that is less than the Winning Threshold, then a Runoff election is held where each voter casts one vote for one candidate in a race between the N+1 candidates with the most votes.
4. The offices are filled by the N candidates with the most votes.
This is essentially the general voting system used in FPTP+Runoff for the single-winner case.
It is similar to Single Transferable Voting (STV), but with a potential additional election, a different winning threshold, and a simpler (FPTP) ballot.