I really like how the game is turning out so far. If you're looking for things to add or improve, I got some suggestions here:
1. The living characters don't really feel 'alive' enough in my opinion. They already got different expressions in contrary to the non-living objects but what could add here is blinking. Characters could blink over time and blink more when they're increasing in size. Other things that could make characters feel more alive is sweating and/or their faces slightly turning red the more they inflate.
2. Like people already suggested below it'd be cool to be able to pinch the hose to hold the air in itself but it'd also be funny if we could use this to inflate the pumps themselves. A toggable tool like clothespins or anything else that could pinch the hose closed would work well with that idea.
3. In my opinion, some of the objects (like for example the banana, swim ring, the fingertips of the glove, the beach ball or the ufo) don't really look like they're inflating and rather simply increasing in size. They could use some more rounding. I love seeing non-spherical objects get rounder over time and eventually become an almost perfect sphere. Additionally for objects like the car, the ufo, the rocket and the beach ball, you could make drawn details like the windows, the lights, the little yellow things the ufo stands on and the top/bottom of the beach ball rather 'shrink' as they inflate.
4. When characters or objects surpass their limits, they could become even more transparent and increase in inflating speed, as its currently difficult to make any big balloon get any bigger.
5. I know this one is rather unlikely but if there were any (non-oc) characters to be added, they could be known characters from different video games or series, like kirby as an example.