The real problem with MWO lights is that they have no proper role.
There's no proper skirmishing or scouting, artillery spotting, and even backstabbing doesn't work well, partially due to map design, partially because approaching enemy without getting detected is pretty damn difficult, not to mention surviving getting back. (Note, i'm completely ignoring faction play, i don't know how it plays nor i care really.) And their cheapness is utterly irrelevant in MWO.
Arguably the doubled armor values (compared to tabletop BT) also cause problems for lights. It is easier to make a heavier 'Mech to out-damage its TT counterpart than it is to make a light do that.
Something like giving 'Mechs different sensor strengths (and types?) and signatures, with lights possessing high sensor strengths and low signatures would make sneak play more usable (except against other lights), while giving them actual battlefield niche. Naturally, a lot of systems should be tailored around these while at it. Missile spotting by lights would be far more reliable than what heavier spotters would offer. Low signature light 'Mechs could easily approach enemies without lights providing sensor data until they're at striking distance. A light in a heavy lance could provide sensor coverage to prevent sneak attacks.
Since assaults would have highest signatures but lowest sensor strengths, they'd rely heavily on other team members to provide targeting data... but once done, they'd also make short work of enemies. (Of course, this kind of system would work best with "auto-target" weapons that aren't really manually aimed beyond general direction and rely on lock-ons; as it is, people don't necessarily even bother with locking a target and just shoot enemies.)
(There'd be non-standard 'Mechs too, like the Black Knight, whose Star League-era version has a Beagle Active Probe by default and so might have above average sensors for a heavy.)
Expanding map to far larger size (say, 10 square-kilometers rather than what they're now) than they're also might help, this would make lights speed more appealing, while making early detection and positioning far more important. Mediums faster than heavies would also gain a role in being more flexible thanks to their speed even if they can't offer quite as much armor or firepower. Assaults on the other hand would also gain usefulness, especially in defensive operations, it is quite possible a big map would spread people out more and thus make an assault likely to outgun its opponents.
Of course, people would probably resort to deathballs still. Multiple objectives spread out across maps might help too, big maps or not: Disable this generator, capture this point, escort this convoy... team who completes most objectives wins the match (and team with most people alive wins if objectives are tied, otherwise it is a draw).
Were i to design a Mechwarrior multiplayer game, i'd probably opt for something like a MOBA: big map (with varied environments), multiple "lanes" with AI-controlled vehicles and infantry/battle armor, simple objective for winning (destroy enemy dropship at their base) but with many ways to get there and kind of sub-objectives to support vehicle/infantry armies ("destroy towers")... Air and artillery support could be called in too, naturally.
This would also include respawning upon death. OmniMechs would have special ability to transport battle armor and select between pre-made configurations at bases but would not allow for standard customization. Standard 'Mechs could be customized but only out of match, and you'd be stuck with what you got, though i'd offer a possibility of choosing a "class" while respawning (similar to dropdecks in faction play in MWO).
I feel this would capture the feel of BattleTech pretty well, drawing upon many aspects, while offering a varied mode with clear roles for various weight-classes, among other things.
EDIT Would need to be 12 vs 12 probably, with one lance (4 'Mechs) per lane usually. A team would have variety of ways deploying and choosing their 'Mechs though, perhaps a light lance as a flexible response unit while each lane would have a few heavies and assaults shoring up conventional forces or perhaps trying to crash through enemies using a heavy cavalry company (running speed of 86 kph+) or whatever else.
EDIT This kind of system would actually give good use for weapons like machine guns (good vs infantry) and LB-X ACs (good vs enemy aircraft and vehicles, if following BattleTech's Total Warfare ruleset for some reason).
