The biggest problem for Harley to produce and sell innovative designs, especially new engines, is the customer base. It really doesn't matter how good it is, the HD customer wants the "potato-potato" V-Twin. The problem for HD is that there isn't enough of them and time marches on, but no one looks at them if they want something other than a V-Twin.
Indian built the Inline 4 motorcycles. The original intent was to phase out the V-Twins. They were, and remain, far more impressive in technology/performace than any V-Twin. They just didn't sell. Everyone wanted a Chief or if they couldn't swing that, a Scout. Now, the Inline 4 is highly collectable and well regarded, but it was a lesson Indian and Harley won't ever forget. You can build it, but if it doesn't sell, it doesn't matter if it's a better bike.
Harley has over sold the 'lifestyle' and now they are pigeon holed as a cruiser builder. No one cares if they produce other bikes, if you want a Harley, you buy a V-twin. If you want an ADV bike or a Sport bike, you don't even consider Harley. In part, because every time you tell anyone you own a Harley, they expect a V-twin and pirate leather.
The suggestion that they create a splinter brand, Acura Vs Honda, for example, is a very good one. Then owner's of the new bikes can say I ride a xxxxxx instead of saying they ride a Harley. I always wanted to build a bike from the V-4 Johnson outboard motor. Fun project and all that. But... I could say I rode a Big Johnson, not a Harley. People would pay more for a Big Johnson.
Harley needs to split off the non-Big Twin designs to a new brand and not directly associate that with Harley. Don't sell it in Harley dealerships, etc. You don't go to the Honda dealership to spend the big bucks on your Acura. You don't go to the Toyota dealership to buy a Lexus. Only Volkswagan has attempted to go upscale with the parent brand. And that's not doing so well for them. Many people simply won't pay $60k for a Volkswagon. Kind of dumb, considering they have Audi and Porche in the house. But that's another thread.