The definition is not that hard I think.
For me a light tank is a CV90120 (tracked) or Centauro (wheeled).
The difficult thing is as you said to define the role of light tanks on the modern battlefield.
For me they just have their roles as well as heavy MBTs.
You are able to deploy them fast and so give lighter formations like the Stryker Brigade a credible punch and are more economic during peacekeeping oversea operations and normal peacetime duty, especially the wheeled ones.
One should just not try to substitute the heavy units complete with light forces.
Both have their roles and you get problems if you concentrate on just one type of unit.
But are these vehicles light tanks, or tanks light on armour?
The 'traditional' role of a light tank is in recon. Just so Waylander doesn't forget history
, in German WW2 recon units tracked and wheeeled recon AFVs were intermixed with good reason. In general the German idea of recon did not include a light tank. The SdKfz 232/4 Puma (so really the current Puma should be Puma II
) was an afterthough from the failed Leopard light tank (only the turret was used). While it is credited with being a good design, in fact it never had a place in German doctrine since recon battalions were not meant to fight it out with enemy armour (even if often forced to do so).
So what happens if one introduces light tanks into the recon element as part of the doctrine? The answer was evident even during the Cold War. The Soviet doctrine stipulated detachment of tanks into forward detachment advance guard. Never mind that in the case of the BAOR the light tanks are really light, the recon troops are likely to report 'tanks' if they see anything with even remotely dangerous weapon. Towards the end of the Cold War, and certainly now, the divisional Russian recon battalions have 6-10 medium tanks in an integrated company a la the US Cavalry. This was in turn based on the expereince of US troops in Europe when the light tanks were being slaughtered in their traditional role by German defenders.
So, the development was based on the premise "if we can't see 'em, we might as well make them more survivable".
For the light tank to return, it needs to reverse this premise, and be better at "seeing them". Giving a lightly armoured vehicle a large gun is not going to enable it's survivability because it still needs long range optics/targeting to enable the gunnery capability.
A vehicle that can add nothing to existing IFV capability outside of a larger gun is just going to invite more attention from all manner of gunners, particularly the REAL tanks IMHO