Space thing
Smash Apprentice
That works at certain percents, but it gets power shielded if you did in the wrong situation I've found. When I'm not sure if that will work, I just opt for the Nair -> jab combo.I haven't really noticed this but I can see your point.
The most common combo I use at early %s is usually N-Air -> F-Smash for a solid 25%.
Utilt -> Usmash is a pretty awesome combo at low%. You can do it out of an Uthrow at low% if the opponent air dodges. Decent mix up since Uthrow -> Uair isn't guaranteed at that point but does cover everything but air dodging.
I've been finding Dtilt -> Fair to be one the more consistent and easy to land kill confirms. It's been more reliable than Dtilt -> Usmash by quite a bit (though they work at different % so it's not like they're competing options).
Not sure what I would call a BnB combo for Greninja that isn't already widely used otherwise.