$\DeclareMathOperator\MCG{MCG}\DeclareMathOperator\Diff{Diff}\DeclareMathOperator\Homeo{Homeo}$Let $M$ be a $1$-connected, closed, smooth manifold with $\dim(M)>4$ and let us set $\MCG(M)=\pi\_0(\Diff(M))$. Dennis Sullivan proved that $\MCG(M)$ is commensurable to an arithmetic group.


*Edit:* regarding A. Kupers' remark on commensurability, here is a nice note
<https://comptes-rendus.academie-sciences.fr/mathematique/item/CRMATH_2020__358_4_469_0/>


* I was wondering if the same type of result holds for topological manifolds and $\pi\_0(\Homeo)$.
* Let us consider the canonical morphism $i:\Diff(M)\rightarrow \Homeo(M)$, what do we know about the morphism induced on $\pi\_0$?
* And more generally about the homotopy groups of the homotopy fiber?
* For which classical manifolds this homotopy fiber has been determined?
* For spheres, this should be well known and related to exotic spheres, and what do we know about complex projective spaces?



Mapping class groups in high dimension
Let me assume that M is at least 5-dimensional.


Sullivan's proof only uses surgery theory and properties of O(n) that also hold for Top(n), so the answer to your first question is yes.


Regarding your other questions: the homotopy fiber Homeo(M)/Diff(M) of the map BDiff(M) -> BTop(M) is subject of smoothing theory. There is a map from Homeo(M)/Diff(M) to the space of sections of the bundle over the d-manifold M with fiber Top(d)/O(d) which is associated to the frame bundle of M using the action of O(d) on Top(d)/O(d). This map is injective on path components and an isomorphism on all homotopy groups. This can be found for instance in the book of [Kirby and Siebenmann](https://press.princeton.edu/books/paperback/9780691081915/foundational-essays-on-topological-manifolds-smoothings-and).


Up to a question about components, this reduces the study of the homotopy fiber you asked about to understanding the homotopy type of Top(d)/O(d) (and the twist of the bundle), which is hard. The homotopy groups of Top(d)/O(d) are object of study in geometric topology for a long time and can be divided into three ranges:


1. For $i\lesssim d$, we have $\pi\_i(Top(d)/O(d))\cong \Theta\_i$, where $\Theta\_i$ is the group of homotopy spheres which is understood in terms of the stable homotopy groups of spheres by [Kervaire--Milnor](https://www.jstor.org/stable/1970128).
2. For $d \lesssim i\lesssim \frac{4}{3}d$, the groups $\pi\_i(Top(d)/O(d))$ can in principle be understood in terms of Waldhausen's algebraic $K$-theory of spaces. Rationally, this can be used to compute that in this range $\pi\_\*(Top(d)/O(d))\otimes \mathbb{Q}$ vanishes for $d$ even and is isomorphism to $K\_{\*-d+1}(\mathbb{Z})\otimes \mathbb{Q}$ if $d$ is odd (that's a calculation of [Farrell--Hsiang](https://mathscinet.ams.org/mathscinet-getitem?mr=520509)). The rational $K$-groups of the integers were computed by [Borel](http://www.numdam.org/item/ASENS_1974_4_7_2_235_0/): $$ K\_{\*}(\mathbb{Z})\otimes \mathbb{Q}\cong \begin{cases}\mathbb{Q}&\text{if }\*\equiv 1 (4)\\0&\text{else }\end{cases}$$ in positive degrees.
3. In the range $i \gtrsim \frac{4}{3}d$ very little is known, but some progress has been made in recent years. For instance, work of [Watanabe](https://londmathsoc.onlinelibrary.wiley.com/doi/abs/10.1112/jtopol/jtp024) (building on ideas of Kontsevich) and [Weiss](https://arxiv.org/abs/1507.00153) shows that there are plenty of nontrivial rational classes.


Later edit: The identification of $\pi\_i(Top(d)/O(d))\otimes\mathbb{Q}$ in the range $i\lesssim \frac{4}{3}d$ mentioned above holds in fact for $i\lesssim 2d$ by Corollary 4.2 of [Randal-Williams](https://link.springer.com/article/10.1007/s00208-016-1504-0) and Corollary C of [Krannich](https://arxiv.org/abs/2002.04647). Beyond this range, progress in even dimensions has been made by [Kupers--Randal-Williams](https://arxiv.org/abs/2007.13884) and in odd dimensions by [Krannich--Randal-Williams](https://arxiv.org/abs/2109.03500).


