I don't play the Adept, but I also agree that Biotic Grenade sounds pretty boring, especially considering that, at the moment, 4 out of the 6 classes have a grenade-type second class exclusive power.
In my opinion, Adept should get some sort of ability that essentially "powers-up" all their biotic attacks (I'll call it Overpower as it is what i originally thought the adept's overpower would do when i heard about it).
So, you would activate this ability, and all your biotic abilities would gain a huge increase in power. For example, they would ignore protections and damage health directly.
Rank increases would increase the how many abilities you can use before it wears off maybe, or how many protections it bypasses.
For example
Rank 1: You can use one offensive ability when activated, and it maybe ignores the enemies armor?
The following ranks would then improve on this, and you could have it branch out into being able to use more abilities beofre it wears off, or for it to ignore more protections.
Not perfect I realise, but its something.
Also, "overpower" would have a really long cooldown, maybe something like 45 seconds? And any biotic abilities you use in overpower mode, share its cooldown. For example: throw has a 5 second cooldown, overpower has a 45 second cooldown. But if throw is used in overpower mode, it has a 45 second cooldown.
Anyway, I think this would be a fun adept ability cause it would kind of give the adept a taste of the "super" biotic abilities they had in ME1, but not over-do it so the game is so un-balanced.
What do you guys think? I'm sure my idea is flawed somewhere lol