Well actually I almost
always Wonder about Heaven xD It seems so unreal, and impossible to imagine for our little human minds

we only have to believe what God tells us about it...
I think the fact there will be no more sin in Heaven is due firstly to us not having physical bodies anymore (or new and incorruptible bodies ?) because something in the animal nature of man has been disturbed since the original sin, and thus may lead to sin (even though most of the sin comes from the mind...) but I believe that first and foremost our minds shall become clearer -and everything such as temptation, trickery etc. will seem to us laughable and gross compared to the immediate and beaming and loving presence of God, so much more preferable, overwhelming and finally unveiled.
Furthermore, God promised to make us Children of God if we take this free choice, and how could we be children of God and sin ? It just doesn't make sense. Because if we are children of God, we are
from God, that is to say we shall share somehow his nature, be in him and him in us, and forever reunited - and nothing in God's nature has anything to do with sin (the sin being the contrary of God's nature). So we can't be from God and sinning...
Yes I see what you mean when you Wonder whether man can conceive any living form which is not human...the risk of anthropomorphism and humanization is always present, because we are so stuck in our human point of view, drowned in subjectivity and our nature...the Bible itself is the Word of God destined to humans -God meant it for us, but it doesn't mean that Man's standard is unique in the whole universe...Yet the Bible tells us that angels have a will, emotions (they can rejoice of be saddened), and intelligence...just like humans, and just like God.
After all, God said He created humans at His resemblance, so there is a divine part in us (for exemple, the moral Law) and this part of God in us connects us with eternity and the whole universe (because God's mark, His "style" is in every created thing) so when one ask whether there could be anything living, self reflective and reasoning, and willing, and emotional, and yet not "human" -this side of Man being also God's, the question would rather be : "how could anything but God work", don't you think ? The natural answer would be "no" !
I am not sure if I made myself clear, that is a very difficult issue

(and so sorry for my English....><)