I’ll venture way out on a limb and fantasize one of them is carbon or glass fiber (e.g. resin impregnated with microfibers, somewhat like the ceramic resin). It would give them something to position against Markforged Onyx.
Yes, it sounds crazy, but users have discussed the idea and its challenges in the past (even experimenting with adding glass fibers and particles like aluminum to resin). For what it’s worth here’s a 2005 research paper from a group that played with adding carbon nanotubes to SL resin.
Unrelated, I spoke with someone (not associated with Formlabs) at the user conference whose startup was working on aligning fibers in liquid during printing using magnetic fields.