If they opt to go the route similar to how Tifa plays on FFVII Remake. They need to untie haste from Greased Lightning, that way we could have interesting mechanics attached to it and have it treated like a resource. Have the speed be a passive thing that is gained through traits. At all the levels MNK gains extra stacks of GL they could receive a trait that simply gives them a passive 5% haste too.
This way, GL would be a pure damage % modifier but would also give access to different oGCDs. For example, base Tifa has Whirlwind uppercut, 1 chi level grants her Omnistrike and 2 chi levels grants her Rise and Fall. Omnistrike I believe is the strongest, but Rise and Fall gives a bigger stagger % increase. The way in which MNKs could use this mechanic could replace the clunky RNG chakra mechanic and tie in with Greased Lightning. At 1 stack we could gain access to TFC, 2 stacks 6SS and 3 could be Tornado Kick etc... Each ability reduces stacks by 1, but the potencies are big to justify dropping. If we were to have a 2nd gauge, we could have it function similar to the ATB system in Remake wherein GL finishers and weaponskills could fill it. At 50 Gauge (Total of 100) we can use Anatman which grants one GL stack on a cooldown (45s?). We could also have the gauge allow access to a couple oGCDs too.
In this instance MNKs would still 'ramp up' for burst but the payoff would be a huge damage dump. I'd tone down the damage so that each GL stack gave 5% damage so that with each stack gone it wouldn't feel like we're hitting like a wet noodle after the burst. I really think the MNK job developer could learn something interesting from how Tifa plays in FFVII Remake. I'm not saying they should make a complete replica of her in this but at the very least I think the haste from GL needs to be passive and only the damage portion of it remains. It's the speed aspect which is clearly causing issues for them developing anything new because every time something is introduced to do with GL it either forces us to go slow or stand still because they just can't balance around the speed of it. Whereas if it the speed was passive then they could balance properly around it because the speed would be consistent and never changing. The devs could really do some interesting things with MNK if they chose to rework it properly instead of clamouring to this dated design.