Also, if we know someone who's definitely going to be facing luffy and his crew and is most definitely defeating a hypothetical gear 5 upgrade that could mean we also see a gear 6 transformation which fully manifests the nika fruits powers which could also mean water powers

Could be kewl!

I doubt it

G5 seems to just combine all the previous Gears

Having other forms in G5 is possible(imo) but having the same form in a different gear, doesn't really sound like smth Luffy would do

No

Gear 5 is just "Gear 5"

For gear 4, from the very start he specified a particular form like Boundman.

The problem with Gear 4 was the excessive Haki use. Gear 5 nullifies that issue, so there's no point for him to specify forms like he did with Gear 4. Gear 5 he can literally just inflate his muscles without the need to blow air like he did in Gear 3rd and 4th. Gear 5 is his objectively strongest form

No