i dont think there will be new instruments for awhile due to the website's limitations and we just added double the instrument count not too long ago. online sequencer doesnt need more instruments, its up to you to be creative with what you're given
Load in Bandlab (another free online music editor)
Change instrument.
Badaboom. Profit.
I get it, I like OS's interface for sketching out the idea of a song, but it's not a tool to let you finish the way you like, necessarily. The two tools work very well in collaboration though, if you like making a midi in OS, BL will split all of the instruments into their own tracks which I don't actually love for tooling around with writing a song, since I like to see how the notes of the instruments are going to interact. But instead of whipping someone who made a free tool because it's not perfect, just use multiple free tools to get the result you want! Sure, it's limited, but OS makes sketching out a song idea very easy, and the more that's jammed in, the harder that would become.