I have been dreaming of an Amiga FM audio card for more than a decade. My MSX2+ has a 2-op YM2413 and I love it ! Now if a FM card could be made for the Amiga clockport, it would be a dream come true !
My choice would go toward the 4-op FM synth YM2151 as this chip has been massively used in the arcade coin-op games in the 80's and 90's so you can find a lot of music for it. I have a collection of several thousand tunes in .mdx format (japanese X68000 computer audio format) for the YM2151.
The YM2151 is quite cheap nowadays (it sells for 3.50 $ on ebay) so I think it could be quite an affordable card, and maybe not so complex as a 060 accelerator to design !