I'd like to know where he is likely to be seen.
Who is most likely to see him?
Why does he appear to some? Does he foretell of something?
I think that the key is to find the right demographic - ie one with the need for a new bogey-figure.
What is the demographic of Slenderman and why was he the right fit for this niche? Slenderman is a completely internet-driven phenomena - it has not yet reached the wider world of newspaper reports, scholarly reviews, or even popular entertainment (even Slenderman comics appears to be internet-based rather than paper-based).
It would appear that Slenderman's demographic (ie who is most likely to see him) is young urbanised technology-savy fantasy fans and paranormal buffs from industrialised western nations.
The need for such a bogey stems from the lack of suitability from the existing bogey-genre: Bigfoot is for the woods, witches are now legitimately spiritual or religious and no longer scary, ghosts can still be scary (perhaps less so nowadays) but are not sinister and lack physical means to inflict physical torment, demons are exclusively for religious-types (although the Slenderman lore does attempt to establish links with demonology), and aliens are somewhat passe and burdened by the 70 or so years of "nuttiness" from that particular subculture.
A new source of fear was needed by urbanised youths for the ritualised performance of legend-tripping (an important if largely unacknowledged cultural rite of passage) - something that differentiates their generation from those previous generations. Slenderman fits the bill - the right bogey at the right time. Quite literally, Slenderman is a creation of this generation, by this generation, and for this generation.
The proliferation of Slenderman artwork (online) also suggests that he fills an important creative function and this should not be overlooked.
In order to create a new bogey of our own I think we first need to find the right demographic that needs a new bogey (traditional bogeys being unsuitable). Perhaps it may be easier to find a needy demographic and rework an already existing bogey.
Perhaps the UM community already has its own unique bogey which we could breathe new life into in order to make it legendary...