An ability that distances yourself from your target: thinking something like being able to push other mechs away (including allies--which would allow teamplay).
An ability to toggle off and on continuous boost(you would be in a state of continuous boosting, even while standing floating still) and you would need to turn the ability off in order to walk. You wouldn't consume fuel and would be able to attack while in this state, but heat generation would be increased. Reasoning behind the fuel and heat is that the mechanism that converts atmospheric gas into fuel would be put into overdrive in order to synchronize fuel conversion to consumption. This was originally an ability that I thought better suited the Nief than the rapid-fire.
The ability to ping targeted spots on the map for enemies. Targeting would work similar to spotting an enemy and enemies would be revealed on radar within a determined radius from the ping point. The mech with this ability would also have a larger radar with a greater view distance on their display. This is for a mech concept I'm thinking of called the Hunter.
I had another concept I was thinking of where a B-class mech would be able to detach a large turret from its body, converting the original body into an A-class mech. The A-class body would be similar to a tech in terms of fighting capability and movement, but would also be able to execute a heavy artillery strike like a long-ranged mortar. In maps like Origin and Uptown with a lot of layers, you could remain as a B-Class, with higher armor and a slower heat rate, but with less mobility and burst capability.
Ideas are becoming progressively far fetched...
I like your grenade buffing idea. Though I think it would be more interesting to have the buffing grenade as a secondary weapon with a short buff duration and the ability be something like increasing the duration and radius of the buff. Add a debuff toggle and a flak cannon and it could be like the tech's burst twin or something.