Mechwarrior 5 is kind of meh without mods. I played with the following:
VonBiome for more battlefields, YetAnotherMechLab, YetAnotherMechlabMechs, YetAnotherWeapon and YetAnotherWeaponClan for mechlab and additional weapons. XenoAI MissionYAML and CoyotesMission to improve mission variety and difficulty.