its simple economics.
Lynx = Cheap, fast, 1 barrel
Panther = Almost as expensive as Tigers, slow, 1 barrel
Tiger = Expensive, really slow, 2 barrel
Lynx are weak but fast and cheap so they can be replaced in battle easily. Tigers are expensive but make up for their price and slowness with armor and firepower. In a pitched battle, people are either trying to spam their enemy with lynx or roll over them with tigers. Panthers were probably included in the game as a stop-gap measure between classes. This can be seen readily in the campaign. In many of the later scenarios the player starts out wiht a number of structures, units and researched items. It usually isnt very far off to research Panthers and most of the time your base is under constant seige. In these instances, panthers do make the difference because their research is so close to being complete and their lifespan is greater than the lynx. In such scenarios Panthers do make a difference, but in multiplayer people either try to win ASAP by spamming lynx or crushing with tigers.
Just because people make a conscious decition NOT to use panthers doesnt mean there is a flaw in the game. They have their uses and a valuable place in the campaign. In multiplayer, people simply choose to otherlook them and save their resources for more lynx or research toward tigers.
-------------
What would a super heavy chassis be? and what comes after that? Ultra Mega Chassis? Amazingly enough, OP2 (and hopefully OP3) are not Total Annihilation clones. Yeah having a Krogoth that stands taller than most buildings, walks around blasting everything with arm-cannons is cool but that kind of stuff isnt for the Outpost "universe". It doesnt fit into the story or gameplay, and would be pointless in a sequel. Wouldnt it be more prudent to just introduce an entirely new type of unit and not just a bigger tiger? Its too formulaic and amateurish. Why not have AIRCRAFT of some kind? Perhaps simply atmospheric gliders, probes or drones that drop a couple bombs or have some sort of laser that does moderate damage? You have to remember that everything in Outpost (especially weapons) is cobbled together from the bits of what was supposed to be a peaceful mission. The Tiger is the best evolution of a weapons platform and is basically a tank. We havent come up with anything better than a tank in real life, except for aircraft. And I imagine in the Outpost universe it is likely that any combat aircraft would be a modified scientific drone. (think army UAV's) The most advanced of which wouldnt be like anything we have in real life since it would have to be built using what they have available. I imagine the largest they could build would be about the size of a lynx not counting wingspan. Maybe that is more plausible?
~~ Sorry my posts get so long, but I find it better to explain thouroughly once than clarifying a dozen times later.