I've been trying to experiment with different stuff after 66C cancel into fireball (on hit and on block).
Heres what I have so far.
http://pastebin.com/B3YUZWxz
Merkava Converting off of 66C xx 214A/B
All damage values calculated with Vorpal ON, against Akaktsuki.
1. Anywhere - CS
66C xx 214A/B xx CS > 6D > j.C > j.2C > 2CC > j.C > j.A > j.B > 66C
~2700 dmg & oki
The above is ONLY if the 66C connects! The combo is cut short due to the fireball using up one of your OTGs! This is not really the combo you want! (explained later on.) Confirming off of 66C varies GREATLY on spacing. The above is the simplest version that works anywhere on the screen with just about any spacing and at just about any height. Depending on the distance you may need to use 214B instead of 214A, though in my admittedly limited testing it would seem that 214A is often the correct choice. Spacing/Height allowing, you can do different routes off of the 6D > j.C (i.e. 6D > j.C > j.A > j.B > 66C)
2. Anywhere - CS
66C (blocked) xx 214A/B xx CS > 6D > j.C > j.2C > 2CC > j.C > j.A > j.B > 66C > 2C > 4B
~3000 dmg & oki
This what you want to happen! The goal is to get you opponent to attempt to mash after 66C is blocked and counter them with fireball. In this scenario the fireball is really the first hit of the combo allowing you to get a 2nd 2C OTG. This gives you some added damage and the joy of beating your opponents mash, which may put them on tilt or otherwise help you condition them. All of the spacing restrictions from the first combo still apply.