Luffy won't get another DF, it's impossible for anyone to achieve that except Teach. Teach achieved it due to two known reasons : Yami Yami and his disproportionate atypical body structure and Luffy lacks both the things.
Also this isn't Naruto where the main character gets super strong because the villain is super strong. Luffy has gotten much stronger in these 2 years, now way we can consider him weak.
That is not true, according to Jinbei Blackbeard is after strong fruits. He never said he was after Luffy's specifically.
And Luffy with two fruits would ruin the series.
Luffy with a gear 4 would ruin it too. that would make it Dragonball all over again.
Let's just leave it to Oda.
First we had all kinds of variations on his rubber attacks
Then Luffy used Rokushiki to his advantage to come up with Gear 2
He invented Gear 3 himself
After that was haki
Now we got Haki + Gear 2 and 3
So Oda is still evolving his fruit without any repetitve random "level up" kind of upgrade.