I've settled on a game idea I'm tentatively calling "Beaver Pitch". You control Barnaby the beaver who has to defend his dam from naughty neighbour beavers who keep stealing logs for their own dams. You have to run up to them and bop them with your tail, while rebuilding parts they have destroyed. I'm thinking that you move faster when you're on your dam, and slower when you're swimming, so the more wood they steal, the harder it is to keep the dam maintained.

Here's a mockup: