If I were doing it, I would first concentrate on true custom work that doesn't look like anyone elses. There's always room for good quality makers of custom products who can first make their name recognition in the custom world, and then expand into production factory product.
Big picture seems to be that there's a glut in the market for good quality grips. There are many options for "normal" grips (i.e., grips that look like a lot of other existing grips.) I can satisfy any whim in exotic woods or other materials with just a phone call or five minutes on the Web. Tuplipwood in double-diamond? No problem. They're cheap too. Hardly justifies the effort in making them.
One niche that's not getting much attention is custom grips for competition shooters and other shooters for whom gun fit and consistent grip is very important. Thin ones for people with small hands, thicker ones for people with bigger hands. Thumb rests in some cases. Other design features for helping the shooter gain a consistent grip. An almost totally unexplored area is the translation of features that work well in competition grips to the more mainstream (and presumably more lucrative) action shooting and defense disciplines.
If you're interested in prototyping some grips for competition, send me a message. I'll be happy to try them, give you an opinion, and return them.
Another area that's relatively unexplored is ergonomic grips, like a grip that alters the feel of the grip angle so it feels like more modern designs that fit the hand better. Guns like the Glocks just point better than the 1911 for new shooters who haven't taken the considerable time to get used to the relatively vertical grips. It's not a natural pointer like Glocks, Lugers, or almost any modern auto you can name.
1911 grips on guns that get used a lot are susceptible to damage by the copious amounts of oil we use on them. You may want to use finishes that can be advertised as resistant to damage by oil and gun solvents.
Most of my shooting is done with 1911s. It's like I'm married to the 1911.