Don't just shift the formant on a single track. Duplicate your vocal. Leave one dry (normal). Shift the duplicate's formant down by -3. Blend them together. This creates a "thickened" lead vocal that sounds like a choir of the same person with different body sizes.
Formant shifting is a powerful digital audio process that alters the timbre and character formant shift vst free download free
When you pitch a vocal up or down, you are changing the fundamental note (the musical key). However, if you pitch a male vocal up too high without adjusting anything else, it starts to sound like a chipmunk. If you pitch a female vocal down, it sounds like a slow-motion giant. Don't just shift the formant on a single track
Graillon is perhaps the most popular free plugin for vocal formant shifting. While the "Full" version is paid, the offers a fully functional Formant Shifting section. Shift the duplicate's formant down by -3