Someone else and I had a similar suggestion to this...I believe it was sierra sonic
Anyways, the way it went was that rather than a charge then shoot, it would be a charging shoot...
Example, I am Vegeta, I knock down Goku, I switch to renzoku, and hold down my button, as I am holding down my button, a bar is rising like a chargup bar, when it gets to full I am stunned until the bar returns to underneath the green line (minimum chargup for other beams), if I choose not to hold down until I reach full capacity, I still must wait until the bar returns to underneath the green line
*Note* I will not begin firing ki blasts until I reach the minimum chargup line, when held until full bar, total ki blasts will equal 8-10 (somewhere around there) if held to a different point I will put out an equivalent amount of ki-blasts (half bar=4-5 etc.)
The time for a bar to reach full capacity would be about the same as a generic beam, possibly a bit more, but the cool down time would be double that
This makes the attack fairly affective, but not unbalanced