Yea keep at it.
And if it doesn't work out don't automatically feel that you NEED to make an RP. Many of the things you are looking for are already there in already existing RPs.
I'll give some examples. And some might take this as a shameless plug to which I answer "poo on you."
I have always wanted a well done TES version of Vampire the Masquerade
In my Vvardenfell RP I have the 3 vampire houses as possible factions. Get some people together and i'll open those up and then you'll have a vampire RP going within an already established RP with people making really wonderful posts. You can become involved in that or remain in the vampire world only.
Anywho, I like that cave idea that you told me about, although you said something about sanity and you would probably need a sanity level or something related. That could get very sloppy very fast. Also, redoing the Bloodseekers would be ok, but you would definitely need a entirely different story/background and that could be hard to come up with, considering you used a lot of main events in Tamriel already. A Db Rp would be awesome too, although I wouldn't make it a free-for-all type of Rp, where you do the hell whatever you want.
DB RP? I already tried a Darkbrotherhood RP and trust me it doesn't work all that well. It's too vague. You can't have assassination being the MAIN point of the story because most of the time people will just be faffing about. And when you do get to the assassination part everyone wants to do a ka me ha mea cliche type thing. The characters usually range from altair copies to cool guy with dual katanas and a trench coat to crazy depraved guy who enjoys hurting people.
On the other hand you could do a Dark Brotherhood and Morag Tong aspect in Circlet Ignited. I already have functioning NPCs, and people with their own goals and people with goals that need to be killed.
As for ideas for a new one, I always wanted some sort of Mafia-related Rp, or gang related with like the different families and stuff, and some sort of "Godfather" figure. That would be cool, although it would require a lot of thinking and work, and of course some sort of big goal or fight or something, free-form Rp's tend not to work out so well. And I would love to help you maintain a new Rp, if you need it, or help you come up with some sort of story/background for an rp.
Hmm a family type RP...where perhaps there were these Houses and these Houses fought in very shadey ways over influence and resources. Truley where could you find such an RP...
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See the issue with making RPs and keepin them alive is that many people often have the same idea but instead of trying to work together they work seperetly and it falls apart.
One RP with a decent plot will last longer than 5 RPs with an amazing plot simply due to numbers. My whole reason for making my brand of RPs was really to have as many people RPing together as possible in order to keep the story going. I HATE it when you get into an RP and it just dies. You never really see an RP with a start, middle and end. You just see a start and then an end. So i say, and this offer is for anyone, if you want to make an RP see if it isn't already out there but perhaps not in a way you expect to see it. If anyone's interested in a vampire or db RP tell me because it is so doable in Circlet Ignited.
:Shameless plug off: