Suppose $\langle X,\mathscr{O}\rangle$ is a topological space and let $\mathscr{O}\_x$ be the family of all open neighbourhoods of $x\in X$. Let $\mathscr{F}$ be the filter generated from $\mathscr{O}\_x$:
$$\mathscr{F}=\{Q\in2^X\mid\exists\_{V\_1,\ldots,V\_n\in\mathscr{O}\_x}\,V\_1\cap\ldots\cap V\_n\subseteq Q\} $$


What I am interested in is under what conditions (if any) can we guarantee existence of a chain $C\subseteq\mathscr{O}\_x$ (may be uncountable) such that the filter $\mathscr{F}\_C$ generated by $C$ is equal to $\mathscr{F}$.



Convergent filters generated by (not necessarily countable) chains
Topological spaces where each point has a totally ordered local basis are known as $\textit{well-based}$ spaces. The notion of a well-based space is a generalization of the notion of a first countable space since the first countable spaces are the spaces where every point has a countable totally ordered local basis. A radial space is a topological space where $x\in\overline{A}$ iff there is some regular cardinal $\kappa$ and sequence $(a\_{\alpha})\_{\alpha<\kappa}$ of elements in $A$ where $(a\_{\alpha})\_{\alpha<\kappa}\rightarrow x$. Every well-based space is radial, and every radial space is the quotient of a well-based space. The papers [3](http://arxiv.org/abs/1412.8701),[4](http://arxiv.org/abs/1401.6519) mention well-based spaces, but I have not found any other mathematical literature that mentions well-based spaces and calls these spaces well-based.


Every first countable space is clearly well-based. Furthermore, if $X$ is a totally ordered set, then $X$ is well based in the order topology and in the lower limit topology.


$\textbf{When the sets $\mathscr{O}\_{x}$ have the same cofinality $\kappa$}$


The well-based spaces where each $\mathscr{O}\_{x}$ has the same cofinality seem to have a special importance among all the well-based spaces.


Suppose that $\kappa$ is a regular cardinal. Then define a $\kappa$-well based space to be a well based space where each $\mathscr{O}\_{x}$ has a cofinal chain of cofinality $\kappa$. Even though the notion of a $\kappa$-well based space seems like a natural notion to study, I have not found any papers that mention $\kappa$-well based spaces. The notion of a $\kappa$-well based space is the correct generalization of the notion of a separable space to $\kappa$-complete spaces.


Suppose that $\kappa$ is a regular cardinal. Then we say that a space $X$ is $\kappa$-complete if whenever $|I|<\kappa$ and $O\_{i}$ is open for each $i\in I$, then $\bigcap\_{i\in I}O\_{i}$ is also open. Completely regular $\kappa$-complete spaces are also called $P\_{\kappa}$-spaces.


If $X$ is a topological space and $x\in X$, then define the character $\chi(X,x)$ of $X$ at $x$ to be the smallest cardinality of a subset $R\subseteq\mathscr{O}\_{x}$ that generates $\mathscr{O}\_{x}$.


Clearly a topological space $X$ is $\kappa$-well based iff $X$ is a $\kappa$-complete space where either $x$ is isolated or $\chi(X,x)=\kappa$ for each $x\in X$. For example, the first countable spaces are precisely the $\kappa$-well based spaces.


For example, the spaces where each $\mathscr{O}\_{x}$ is generated by a chain of cofinality $\aleph\_{0}$ are precisely the first countable spaces. It turns out that the $\kappa$-complete spaces $X$ where each $\mathscr{O}\_{x}$ is generated by a chain of cofinality $\kappa$ behave very similar to the first countable spaces. In fact, the basic theory of $\kappa$-complete spaces is very similar to the basic theory of topological spaces since basic results about topological spaces often generalize the results about $\kappa$-complete spaces, ad the notion of a $\kappa$-well based space is the correct way to generalize the notion of a first countable space to $\kappa$-complete spaces. The papers [1](https://eudml.org/doc/213209),[2](https://eudml.org/doc/213209) develop some of the basic theory of $\kappa$-complete spaces and generalize some facts about topological spaces to $\kappa$-complete spaces. 


$\kappa$-complete spaces can easily be generated by topological spaces. Suppose that $X$ is a topological space and $\kappa$ is a regular cardinal. Then define $(X)\_{\kappa}$ to be the topological space with the same underlying set as $X$ but where $(X)\_{\kappa}$ is generated by the basis consisting of sets of the form $\bigcap\_{i\in I}U\_{i}$ where each $U\_{i}$ is open in $X$ and where $|I|<\kappa$. Then $(X)\_{\kappa}$ is always a $\kappa$-complete space.


The following results are a rather straightforward generalizations of results about first countable spaces, but they illustrate that the notion of a $\kappa$-well based space is the correct generalization of the notion of a first countable space. 


$\mathbf{Theorem}$ Suppose that $X$ is a $\kappa$-well based space. If $A\subseteq X$, then $x\in\overline{A}$ if and only if there is some sequence $(a\_{\alpha})\_{\alpha<\kappa}$ with $a\_{\alpha}\in A$ for each $\alpha<\kappa$ converging to $x$.


$\mathbf{Theorem}$ Suppose that $X$ is a $\kappa$-well based space. Let $Y$ be a topological space and let $f:X\rightarrow Y$ be a function. Then the following are equivalent.


1. $f:X\rightarrow Y$ is a continuous function.
2. $f:X\rightarrow(Y)\_{\kappa}$ is a continuous function.
3. Whenever $(x\_{\alpha})\_{\alpha<\kappa}\rightarrow x$, we have $(f(x\_{\alpha}))\_{\alpha<\kappa}\rightarrow f(x).$


$\mathbf{Theorem}$ Suppose that $X$ is a $\kappa$-complete space. Then the following are equivalent.


1. Every open cover $\mathcal{U}$ of cardinality $\kappa$ has a subcover of cardinality less than $\kappa$.
2. Every sequence $(x\_{\alpha})\_{\alpha<\kappa}$ accumulates at some point.


$\mathbf{Theorem}$ Let $X$ be a $\kappa$-well based space. If $(x\_{\alpha})\_{\alpha<\kappa}$ is a sequence that accumulates at some point $x$,
then there exists a subsequence $(x\_{j(\alpha)})\_{\alpha<\kappa}$ where $j:\kappa\rightarrow\kappa$ is strictly increasing but where $x\_{j(\alpha)}\rightarrow x$.


$\mathbf{Theorem}$ Suppose $X$ is a $\kappa$-well based space. Then the following are equivalent.


1. If $\mathcal{U}$ is an open cover with cardinality $\kappa$, then $\mathcal{U}$
has a subcover of cardinality less than $\kappa$.
2. If $(x\_{\alpha})\_{\alpha<\kappa}$ is a sequence, then there is some strictly increasing map $j:\kappa\rightarrow\kappa$ such that
$(x\_{j(\alpha)})\_{\alpha<\kappa}$ converges to some point.


We therefore conclude that the $\kappa$-well based spaces are spaces that one can study simply by considering the $\kappa$-sequences and their convergence. Furthermore, one can get by with thinking about $\kappa$-well based spaces n the same way that one thinks about first countable spaces. 


I hope I did not stray too far from the information which you are looking for in this answer.


$\mathbf{References}$


1. [$\omega\_{\mu}$-additive topological spaces](https://eudml.org/doc/213209). Giuliano Artico; Roberto Moresco
Rendiconti del Seminario Matematico della Università di Padova (1982)
Volume: 67, page 131-141
ISSN: 0041-8994
2. [Remarks on some topological spaces of high power](https://eudml.org/doc/213209). Roman Sikorski
Fundamenta Mathematicae (1950) Volume: 37, Issue: 1, page 125-136 ISSN: 0016-2736
3. [Convergence properties and compactifications](http://arxiv.org/abs/1412.8701). Robert Leek. University of Oxford
4. [An internal characterisation of radiality](http://arxiv.org/abs/1401.6519). Robert Leek. University of Oxford


