No, I agree with MaskedFeraligatr in that if Terapagos has another form, it is likely gonna be another Terastalized State form (Like Ogerpon)
With that said, I really don't see the 19th Tera Type being just typeless. Consder that its crown looks like this-
View attachment 573466
- and that the Pokémon most likely to be associated with it also has all 18 types represented within its shell. I highly doubt that 19's visual style would involve "let's display all other types!" if it was the complete opposite by lacking a type. Imo it'd also be kind of counterproductive to focus an entire gen's mechanic around changing types and caping it off with "Ok now here's a special Terastal that has
absolutely zero type interactions!" since it just disconnects itself entirely from the rest of the gimmick
If anything, considering the previously mentioned all-type display and the fact it was shown to boost every move in Baxcalibur's moveset, I'm expecting this to somehow allow you to flux between the other 18 types. Not exactly sure
how, though