I like the idea of being able to do anything myself.
Me too.
I'm building BW as the level design tool I would have enjoyed when I was a kid - easy enough to spit out a quick dungeon, but with enough flexibility to pretty much build your own game. Striking a balance is tough, but I have some plans, mostly revolving around unlockable features (start out with basic tools and preset objects, then unlock deeper-level modification tools later).
As for the baseline playstyle, I'd say Zelda is a good example of what I'm going for. It is also a good example of how changing the average enemy can change the feel of the game, or an area within the game, while leaving the player controls unchanged - some enemies can swarm you and die in one or two hits, others (usually minibosses) require you to play defensively and wait for the right moment to strike. Dark Souls uses the latter type as its basic mook, while Super Mario is composed almost entirely of the first. Any game with a level creator and availability of both enemy types will produce examples of either kind of game, or a mix.
This is a nice post detailing different "feels" for fighting systems, which I made use of while designing the combat system. I want it to be possible to make worlds that occupy all parts of the spectrum.
Transformations could be an issue as far as making every world feel like a different game, but I hope to make the player character's controls robust enough that most world builders won't feel the need to bother with it, or make a handful of default transformations that will be available to all players, making the game more coherent. Unique transformations could be fun occasional diversions, sort of the way they were used in Banjo-Kazooie.
A chargeable basic attack could increase the potential application of the default avatar - you can hack and slash through weak enemies, but stronger enemies might have heavy armor and will shrug off non-charged attacks, forcing you to plan ahead more while fighting them.