I'd like to see a hot wheels-like sentai someday, but is never gonna happen.

What if they do a Gokaiger 10 years after movie, that has living Ranger Keys in it? Like a Sentai made of living Ranger Keys?

Would Turboranger be a mystic arms sentai ?

Nope. Their stuff is primarily technology based.

Did Gunmajin ever show up or get mentioned in Gokaigar? I don't see him i the Legendary war clips or in the various transformation clips for it.

Gunmajin wasn't treated as a ranger or extra hero in Gokaiger. So there's 1 of 2 possibilities that could have happened.
Either he was destroyed during the mecha fight, or he just didn't return for the great war (As he leaves Earth after Ohranger).

Where can I find the rumors of Sixth Rangers for Lupinranger VS Patranger?

In the Lupin/PatoRanger rumors thread.

Which grunt as of 2018 has the best design?
Choose only one.

Personally I choose the Podermen from Lupin VS Pat.

Grunts?
I would say the Dustlers from Denziman.