Decision gathering process in target-detection wireless sensor networks (WSNs) has an essential impact on the overall performance. Among the different decision gathering approaches, a recently proposed scheme, denoted by index modulation based WSN (IM-WSN), has shown a significant enhancement in the decision error rate (DER). The IM-WSN scheme exploits the concept of IM to convey sensing results from distributed clusters in cluster-based WSNs. In this work, we aim at further enhancing the decision accuracy of IM-WSN by improving the transmit diversity of each cluster. Specifically, two schemes, referred to as group-based IM (GIM-WSN) and combination-based IM (CIM-WSN), are proposed. Both schemes rely on increasing transmit diversity by allowing more than one node to act as cluster head and forward sensing results to the sink node. The average decision rate is formulated in a unified closed form expression for both schemes. In comparison with the state-of-art schemes, simulation results show that the proposed schemes exhibit better performance in terms of DER without additional complexity or energy consumption.