What do you mean by "if you get more than 5 keys"? More than 5 PERFECTs? Or should it be valid for GREATs and GOODs? I think that it should be for PERFECTS only or it would be too easy to make combos. I'll add that feature for sure, but don't know when as the guy doing the artwork (Zenzuke) has his HD broken and has to buy a new one.
'Perfect's and 'great's should add on to any combo, but 'good's should be made so that it breaks the combo instead of adding on to it. It would add a more challenging task to get a high combo count all the while keeping the difficulty of doing so at a reasonable limit.