Yeah, evaluate your pricing. I think it's a great deal already, so on the one hand I'd be sad to see the price go up, but I think it's worth it.
You can also count on me for purchasing the second collection as soon as I know about it.
I also don't know how much of a percentage Patreon takes, but maybe leveraging that to get more monthly income would be beneficial?
And so that you don't feel bad about posting it :D https://www.patreon.com/kenney
EDIT: I'm not an artist, but I wish I was. I don't know how you'd feel about possibly commissioned work that would be released? Like if you said 'if I get $x in support for additional __ (platform, road, roguelike monster) assets I'll do them,' that might be a way to focus in. But it might also cut down on the creativity you're allowed, and the 'masses' might want to do a bunch of stuff you'd rather not do anymore of.
Sweet! These Roguelike sets you're doing should be awesome for prototyping.
Question on your website - how come the preview link doesn't show the second picture, such as you display on OpenGameArt: http://opengameart.org/content/roguelike-caves-dungeons-pack They're great for providing perspective!