I never really thought of the .30-30 as a very good case to wildcat (even in my looney days of cartridge conversion). The .30-30, .25-35 and .22 Zipper pretty much cover all the bases that I consider useful.
A second problem was the rifles typically chambered for the .30-30 also limit what can be done due to basically weak actions or tubular magazines (or both).
However, there was one conversion I did try and find extremely useful......the .35-30. This is sort of in the opposite direction you are talking about (necked up....not down), but is very easy to do and filled a perceived need I saw at the time.
I always liked the Winchester 94 rifles for woods use, but didn't like the .30-30 as well as other lever action rounds I'd used. The .32 Special came pretty close, but wasn't in the same class as a .35 Remington for close range work.....but the .35 Remington wasn't available in the Model 94.
With a .35-30 I could duplicate .35 Remington ballistics in a Model 94 rifle by simply necking up the .30-30 case to .35 caliber. This was great in the early 1970's (when I was experimenting).
Since that time the .375 Winchester was introduced which accomplished the same objective with a factory round (and even more bore size....a good thing) and makes the .35-30 a bit less attractive. However a rifle looney doesn't NEED a really good reason to experiment, so the .35-30 might be just what you are looking for.
