Something I have thought about recently has been those 'mechs from TRO 3050 that were meant for the "last war", so to speak. Those designs that would have excelled in a 5th Succession War where the Clans never arrived, and where the gradual evolution and application of tech would have been more forgiving. Obviously, we see rapid changes in design philosophy by 3055 and 3058 with the expanded use of gauss rifles -- even double gauss rifles on a design. Considering how many of the writers had military experience, the idea that arms manufacturers would make imperfect designs is a nice touch for the universe, and I wanted to explore the idea.
I think one of the more obvious to me is the Axman -- either the close range brawler 1N or the skirmisher 2N (a better Catapult or Crusader) -- and could easily tear up most 'mechs on the battlefield in a pre-Clan era. Another would be the Wolf Trap, which is overall poorly regarded, but is just a smaller Centurion. You have the under-gunned Mauler, but I can't help thinking it would be a menace against fast-moving light 'mechs and a nightmare for aerospace fighters.
What other examples are there that come to mind? And not just 'mechs: I meant for vehicles and aerospace fighters as well.