Take the evolution of Ichigo's hollow manifestations. This can at least give us some clues on how powerful Ichigo's hollow side can be.
Thr first one that manifested on his vizard training was quite raw, lizardous form. It has high regenerative powers, can use zanpakutou (unlike Arrancars using sword-like things) and has a Cero that warranted seriousness from his vizard sparmates. Note that the face of this hollow form was consistent with the very first hollow mask of Ichigo after training with Urohara to regain his powers. And his Cero comes from the fingertips.
The second one was more humanoid, but the mask's form changed. It instead became sort of like a skull-esque one with horns. The mask's design (e.g. lines and markings) differed from the previous. Ichigo retained his body but a few parts didn't - wrists, finger and toe nails, hair. Oh, and he easily tore apart Ulqiorra before the distraction by Ishida and Orihime. And the Cero now comes from the horns.
Most likely the next form may involve another change, since his iterations suggest a Darwinian evolution.
If this hollow manifestations suggest power, it conforms with Ichigo's fight with his hollow side - it can manipulate zanpakutou, it has an equally high supply of energy much like Ichigo, and can regenerate fast. In short, it's as powerful as Ichigo. And to think Yoruichi once said Ichigo can compete at a captain's level.
Now going back. There are some forms a menos has to evolve to in order to become Vasto Lorde. First, the basic Menos. Brainless and elongated. Next stop is a "gillian", a few brains but with untapped potential (see Espada #9).
The following evolution is an "adjucha". It is a humanoid-form hollow with a "mask". Powerful and dangerous, this is Aizen's pig farm, using the Hougyoku, making the Arrancars.
Finally, the cream of the hollow crop, Vasto Lorde. All we were left in the anime is a silhouette of a Vasto Lorde. And the manga describes the Vasto Lorde as "insanely powerful".
Obviously it has horns and a big hole in the chest. Ichigo's recent form has horns AND a big hole in the chest. Almost none of the Espada got horns (unless you count Ulqiorra but I think his third form ARE horns but are quite small) and their holes come from different places.
I have to concur at some point, Ichigo's hollow side would be revealed as a Vasto Lorde. How he ended up in the body of a son of a delinquent shinigami, would be Kubo Tite's ball to play.