Yep, there are a little less than 20 placeholder mega stones that will eventually be something. I haven't decided what yet though!
IMO Zygarde is fine as is, but Ultra Necrozma will need some changes seeing as Z-moves aren't in.
I've never seen that one before, where were you when you left it running?
If you're looking for some ideas for the Megas, I got some:
Gen 1: Parasect, Hypno
Gen 2: Xatu, Sudowoodo, Dunsparce, Octillery
Gen 3: Delcatty, Ninjask/Shedinja, Chimecho
Gen 4: Wormadam/Mothim, Carnivine, Lickilicky
Gen 5: Liepard, Zebstrika, Seismitoad, Scolipede, Maractus, Zoroark, Swanna, Beheeyem, Cryogonal, Druddigon, Bouffalant, Hydreigon, Volcarona
Gen 6: Diggersby, Pyroar, Gogoat, Pangoro, Furfrou, Barbaracle, Klefki, Avalugg
Gen 7: Toucannon, Crabominable, Oricorio, Ribombee, Mudsdale, Araquanid, Shiinotic, Salazzle, Comfey, Golisopod, Pyukumuku, Komala, Dhelmise
Went for a mix of Pokemon that could use some Mega help, and some that would be cool to see (just like Game Freak lmao)
As for the Ultra Necrozma changes, I'd say make it a Mega Evolution, increase its BST to 780 and buff Neuroforce, but still make it have to hold a Mega Stone so that it's at least somewhat balanced.
And for the crashes, I was usually in the overworld, mainly happening in Petalburg Forest, though it did also happen in battle a lot.