Can Hydronium Act as a Ligand? Can hydronium function as a ligand? For instance could there exist $\ce{[Al(H3O)4]^{+7}}$ or $\ce{[Fe(H3O)6]^{+9}}$ cations? I asked my chemistry teacher and he said that he's never encountered a coordination compound with cationic ligands (however I know that there are because $\ce{NO+}$ can act as a ligand). What are the properties of these cations, if they exist? How are they synthesized? And finally, how would you name these complexes?
This is not the answer OP would expect because I couldn't find metal complexes of type $\ce{[M(H3O)_x]^{y+}}$ (the reason for non-existence(?) is mentioned in the comments) but I did find a complex of an organic compound where hydronium ion act as a ligand i.e a complex of 18-crown-6 and hydronium ion(emphasis mine):

The observation that cyclic crown ethers can bind cations with high
affinity and selectivity has had a tremendous impact in chemistry. The
complex between 18-crown-6 (18c6) and the hydronium ion, $\ce{H3O+}$,
is a prototypical representative of this important host−guest paradigm
and has been the subject of numerous experimental and theoretical
studies. The computational evidence presented so far has favored a
structure with three linear $\ce{O−H···O}$ hydrogen bridges as the
lowest, stable minimum, whereas a bifurcated arrangement has turned
out to be a transition state for $\ce{H3O+}$ rotation 4−5 kcal/mol
higher in energy.


The closest metal complex I could find where hydronium ion is present as an entity is hydronium zinc(II) pyrazine-2,3-dicarboxylate(again not the type of complex OP is looking) where 2,3-PZDC (μ-pyrazine-2,3-dicarboxylato-N,O,O′) act as ligand(emphasis mine):

The structure of dihydronium
[catena-bis(μ-pyrazine-2,3-dicarboxylato-N,O,O′)zinc(II)],
$\ce{(H3O)2[Zn(2,3PZDC)2]}$, is composed of polyanionic ribbons of
zinc(II) ions linked by double bridging 2,3-PZDC ligand molecules.
Each ligand uses an N,O bonding moiety formed by one carboxylic group
[$\ce{Zn–O}$ 2.071(2) Å; $\ce{Zn–N}$ 2.184(2) Å] and a monodentate oxygen atom of
the other carboxylate group [$\ce{Zn–O}$ 2.092(2) Å]. Coordination around the
zinc(II) ion is strongly distorted octahedral. Hydronium cations
$\ce{H3O+}$ link the ribbons by hydrogen bonds.

References

Hydronium Ion Complex of 18-Crown-6:  Theory Confirms Three “Normal” Linear Hydrogen Bonds
Michael Bühl, Ralf Ludwig, Rachel Schurhammer, and, and Georges Wipff,
The Journal of Physical Chemistry A 2004 108 (51), 11463-11468
DOI: 10.1021/jp045879+
Michal Gryz, Wojciech Starosta &amp; Janusz Leciejewicz (2005) &quot;Doubly bridged molecular ribbons in the structure of an ionic complex, hydronium zinc(II) pyrazine-2,3-dicarboxylate&quot;, Journal of Coordination Chemistry, 58:11, 931-935, DOI: 10.1080/00958970500055492