Re. integrated wiki: when i first saw fossil (Christmas break of 2007) two features made it a killer app for me: wiki and hosting as a CGI. The wiki aspect has long since taken a back seat to the so-called "embedded docs" feature, where the docs live in the source tree and become first-class SCM citizens. However fossil is, to the best of my knowledge, still the only SCM which is absolutely trivial to host as a CGI, which means it can be hosted on cheap shared hosters just as easily as it can on one's standalone VPS.
As far as "what comes after git," though: git is the SCM needed by the 0.1% (or fewer) of the largest, most-active FOSS projects. Imagine the Linux kernel source tree if its SCM could not remove dead branches - it would quickly become uncloneable under all of that weight. Fossil is not designed to scale to projects of that size. Fossil is, however, an ideal SCM for that 98%+ of remaining projects which fall into the size categories of personal/small/medium.