Nothing

Opting

Pooting

Quieting

reminiscing

Sledding

tripling

Underling

Viewing

Wishing

accepting
bettering
constructing or committing
defying
empathizing or energizing
fascinating
generating
happening
initializing
jiving
kidding
laughing or listening or loving (L has so many good ones!)
moving (I guess "Mingling" would be best, but these are all first thout words)
nourishing
obsessing
playing or promising (P has a bunch! another notable mention, procrastinating... "Procrastinators are the leader of tomorrow")
questioning
respecting
sensing
togethering
unifying
vocalizing
winning
xylographing (xylophone was my first thought, so I tried to make it a verb...don't know if that's a word or not...but I'm going with it!)
yammering
zooming

First thought A-Z ending in ING ... first draft, definitive list

Yellowing

zipping

Abandoning

blushing

Caressing

deterring

Entertaining

flogging

Groping