Similar to how Snake can alter the way he chucks his grenades, is it possible to change the trajectory of the grenade egg? It feels slow to use, requires me to face opposite the opponent, and the egg itself also travels slowly.
Sakurai mentioned in the direct that the grenade is "football shaped" so it will bounce "in an unpredictable manner." I've tried getting it to bounce differently by jumping and shooting the grenade at different times, but it seems to always start bouncing slow (and then speed up at different rates depending on the angles).
I also tried holding B vs. pressing it quickly, which will work for things like Isabelle's fishing rod/palutena's side-B/etc...but to no avail with Banjo.
If you want to quickly get a grenade in your opponent's face, I'd suggest doing it like this:
When you do the down-B in the air, you stall for a second. If you point the control stick to match the trajectory of the grenade and then press Z to grab it in the air, you can immediately throw it!
The timing and movement are a little bit tight, so it might take some practice, but once you have it down to muscle memory, it should be pretty simple.
Here's the grab in slow motion:
You can also use this for recovering! As these twitter videos show, it's going to be very difficult to gimp a Banjo player that knows how to use the grenade for themselves:
In fact, the Beefy Smash Dudes even made a game out of it!