In this paper, a generalized MIMO Spatial Multiplexing (SMX) system with a new Index Modulation (IM) domain called filter IM domain is proposed. This filter domain generalizes the time and frequency IM domains. In particular, a Filter Shape IM (FSIM) is used to virtually transmit a part of the information bitstream by indexing the filter shape varying at the symbol rate. This scheme permits to achieve high Spectral Efficiency (SE) since it uses all the available time, frequency, and spatial resources. The proposed transceiver can be easily reconfigured to operate as a conventional MIMO SMX transceiver. Compared to the existing systems, the proposed system achieves the highest SE gain with the help of IM and MIMO multiplexing gain. The theoretical lower bound is derived to characterize the SMX FSIM system performance. The results reveal that SMX FSIM with 2 filter shapes only outperforms by 3.5 dB up to more than 12 dB the equivalent SMX QAM system at the same SE. Furthermore, a complete analysis in the sub-THz indoor environment with RF impairments is provided for the proposed system, SMX QAM, and Generalized Spatial Modulation. Finally, the results show that SMX FSIM with a linear receiver has better performance and robustness to phase noise, lower transceiver cost, higher SE/energy efficiency gain, and lower power consumption.