There are 4 options that fit all the criteria of contraversal, humanoid, faction iconic, and completely unseen to date.
We know it won't be the Urbanmech (it's engine isn't even in game), or the Panther (that was denied in the podcast). So the first choice is the Hermes, it has 3 variants at the time of 3050 (even if some of them were mothballed). It's speedy, all energy and quite a tiny profile much like the current Spider. The next is the Firestarter, it's mostly energy hardpoints and the stock configurations are going to be very lackluster. With heavy energy hardpoints, some minor ballistic, and jump-jets it could be a contender. However it's not affiliated with a house and they were typically only used under special circumstances (involving fires).
The Wolfhound is an toss up, whilst it is in production, it is iconic to Merc unit's the Wolf's Dragoons and Kell Hounds, it just doesn't have the variants to really support it (they all revolve around the same set-up just moving rear-arc weapons forward etc.). The Javelin, seen as a House Davion scout mech, involved the same roles as the Steiner Commando. It was a light weight hit and run machine, with jump-jets and speed it could get in and out quite effortlessly and heavily incorporated into the Federated Suns war machine. There are 3 notable variations, from the SRM6 10N, to the 4ML 10F, followed by the Helm Core revamp 10P.
The Javelin is also a prime candidate because of recent plugs that the Jenner hero mech "Oxide" is feeling weak... Whilst not the strongest solo light mech, as a wing-man style machine hunting other lights the 4SSRM's really tear apart smaller targets, luck-fully it didn't get jump-jets or it would be the new Jenner you would only see. However why should it feel weak now? Because their comparing it to something that's most likely out performing it, whilst lighter, the Javelin does come with Jump-jets with gives it the trump card on the "Oxide" all missile light.