THEMIS, a 90 cm solar telescope, offers spectropolarimetric observations in the VIS and NIR spectral ranges through a long-slit spectrograph. Now, an image slicer-based Integral Field Unit (IFU) is being developed as an evolution of the long-slit set-up. The IFU will allow observing a 2D Field-of-View (FoV) and several spectral regions simultaneously. Integral Field Spectroscopy mitigates very well the loss of information generated by the atmospheric seeing over a single long-slit and is more suitable for 2D evolutionary studies of solar phenomena. The IFU optical design inherits the concept of the IFU currently installed on the GRIS spectrograph at the GREGOR telescope and has been adapted to THEMIS. Also, we took advantage from the image slicers manufacturing process evolution since the GRIS IFU was created. First, the spatial resolution is increased by reducing the width of the slicer mirrors from 100 to 80 μm. Second, the FoV is increased by changing the slices number from 8 to 16. In terms of optomechanical integration, the slicer mirror unit is placed at the telescope focal plane. The available volume is limited and has constrained the IFU elements position and the mirrors focal length. The design strategy generates 16 different pupil planes that are superimposed and keeps the optical path constant to allow interchangeability between the new IFU and the long-slit mode. The most challenging condition was keeping the pupil plane position unaltered since the input beam is not telecentric. This contribution presents the IFU optical design and describes the challenges faced in design phase.