Off-limits? Errrrr....
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=Cthulhu+Mythos&x=21&y=23
http://www.nightshadebooks.com/cart.php?m=product_detail&p=201
http://miskatonicriverpress.com/
http://chaosium.com/callofcthulhu/cocfiction_list.html
(That last is by no means a complete list of the books in the series.)
http://www.hippocampuspress.com/mythos-and-other-authors/fiction
(Not all of these are related to the mythos, but a fair number either are, or -- in the instance of collections -- dip into it now and again.)
http://lovecraftzine.com/2011/10/12/matthew-carpenters-upcoming-mythos-books-list/
And there are writers such as Laird Barron, W. H. Pugmire, Thomas Ligotti, Caitlin R. Kiernan, William Browning Spencer, Michael Shea, Brian Stableford, Michael Cisco, Adam Niswander, etc., who continue to contribute to this sub-genre (see, for instance, S. T. Joshi, ed.,
Black Wings: Tales of Lovecraftian Horror, or Ellen Datlow's
Lovecraft Unbound) in new and original ways, doing HPL proud by utilizing his concepts, sometimes his settings or figures, in ways which both capture much of his originality, and combining it with their own unique visions to produce something which is anything but a rehash or earlier material or a rip-off of Grandpa Theobald (a.k.a. H. P. Lovecraft).
In fact, at the end of last year, I was asked by Wilum Pugmire to do an introduction for his (then) latest, an interconnected series of prose-poems, vignettes, and short stories which were direct references to Lovecraft's
Fungi from Yuggoth sonnet sequence... things which wove in and out of what Lovecraft had done, yet were completely original and blended horror, beauty, pathos, perversity, decadence, and dream in an often exquisite prose style; a book which I would love to see have a wider expsore than it has to this point (it was released in a very limited hardbound edition which had practically sold out prior to publication). A strange piece, indeed, and quite enigmatic... but in it (as in so much of his work) he wears his indebtedness to Lovecraft very proudly, without being simply a
pasticheur.*
That is the danger in following in HPL's footsteps: not making direct references -- something writers as diverse as Doctorow and Lumley do -- but managing to be original in one's use of the material. However, given the amazing number of writers who do manage to successfully do this, it is anything but impossible... just
extremely difficult.
So, my advice would be: If Lovecraft inspires you, then by all means use his work as a springboard; just follow your own muse in doing so, and you will thereby remain both true to yourself, and to a gentleman who didn't want people slavishly following his lead, but finding their own voice, and who encouraged this at every turn.
*For those interested, the title of the volume is
Some Unknown Gulf of Night, and it is, from my understanding, soon to be released in a trade paperback edition... though I have no idea concerning the price. And yes, I am proud to have been a part of this little book.