The simplex category $\Delta$ is defined as the category of (non-empty) finite ordinals and order preserving maps. Furthermore, a simplicial set $X$ is defined as a contravariant functor $X: \Delta \rightarrow \text{Set}$. 


I am interested in the possibility of generalizing the notion of a simplicial set by considering the category of infinite ordinals $\mathcal{O}$, if such a thing exists, and then defining an $\infty$-simplicial set as $X^{\infty}: \mathcal{O} \rightarrow \text{Set\*}$, where $\text{Set\*}$ is the appropriate adjustment of the category of small sets $\text{Set}$ such that mapping from objects in the category of infinite ordinals will still satisfy contravariance.


Assuming such a generalization exists, how does this affect the geometric definition of an $(\infty,1)$-category? Is there no longer a capturing of the geometric model desired from simplicial sets or can something more general than an $(\infty,1)$-category be defined? It would also be interesting to see how this would change $(\infty,1)$-functors between simplicial to sets to some other type of functor between $X^{\infty}$ sets. What would natural transformations look like, if they could still be defined properly?


EDIT: I asked this question to a graduate student who is doing work in $\infty$-categories, and he said that you would not get the same geometric model you want by quasicategories if you allowed infinite ordinals. Does this make sense to anyone? He said, that by taking the geometric realization "you probably wouldn't get anything back", but I don't really know what to make of that.


EDIT 2: This question has really been interesting me, and I can't find anything on it in Lurie's Higher Topos Theory or any other literature I have looked through. It seems like any time an author introduces the idea of a simplicial set to aid in defining quasicategories they don't think about possible variations on the simplex category that might change the entire construction they are making into something completely different. Let me know if you want a more specific question to answer!



How would generalizing simplicial sets affect $(\infty,1)$-functors between $(\infty,1)$-categories?
It seems to me that there should be some not too nebulous
motivation for such a question. Rather than answer the
question as stated, I'll give an open problem along the 
same lines. It starts with an old and much neglected paper:
Daniel M. Kan
Semisimplicial spectra
Illinois J. Math. Volume 7 (1963), 463-478.
That gives an analogue of based simplicial sets that allows
for simplices of negative dimension and is designed to give
an alternative definition of spectra. There are problems 
with the smash product and there are several later papers that 
flesh out the theory (Kan and Whitehead, Burghelea and coauthors).
I would be curious to see how this definition fits into the modern 
world of spectra. There is a notion of Kan semisimplicial spectrum
(opus cit) and it seems very natural to wonder if there is an
interesting version of stable quasicategories sitting as a 
subcategory of the category of Kan semisimplicial spectra.


