I don't think any of the forks of bitcoin have a promising future, because the development of the Lightning Network will render them unnecessary. I also think they sow market confusion and give the impression that bitcoin is actually inflationary. It's probable, though, that they will rise in value during the next bull run.
Exactly. I also believe that the Lightning Network will render many Bitcoin forks completely useless. The main reason this Bitcoin fork craze started, was to create a scalable version of Bitcoin with an increased blocksize (apart from greed). Bitcoin Cash was the one crypto that started it all, but has taken a quite risky path that's been largely untested. On the other hand, Bitcoin's Lightning Network is being tested each day to become a secure, and decentralized second-layer protocol for instant and inexpensive micropayments.
I think you're right about the value of Lightning Network, but you're short-sighted to think of it only in terms of Bitcoin. With cross-chain atomic swaps, BTC, BTG, and LTC can be exchanged through Lightning Network and all three types of coins can be transacted.
The base code for BTG on Lightning Network (LND) is already in place, but just porting some code and saying "LN exists!" is easy. Creating all the necessary pieces for a living LN ecosystem is much harder - node software, mobile clients, user-friendly GUIs - this take a lot of work and a lot of libraries need to be made available to other developers. That's what the BTG devs are working on.
Current development is focused on the prerequisites for a better future - enabling Compact Block on the full nodes, porting btcsuite, bringing in Neutrino. There are a lot of moving pieces and dependencies to build a working LN ecosystem!
If [Lightning Network is] successful, then there would be no use for an "improved version" of Bitcoin. This means that Bitcoin Gold, Bitcoin Cash, BitcoinZ, and many other BTC forks would have no reason to exist...
You're right that Bitcoin Cash was created to "improve" Bitcoin with a block size increase. Bitcoin Gold, however, was not created in response to a scalability concern.
It was created to provide a more decentralized alternative. Go to your favorite computer retail store chain, or your corner store - can you buy an ASIC and start mining BTC or BCH? Probably not. OK, can you buy a GPU to mine BTG? Yup.
Better yet... do you know anyone who already has a graphics card - maybe a video designer, probably a gamer? Well, they need to spend $0 to buy equipment for mining BTG. They can just install software, join a pool, and start mining.
That's how you lower the barriers to entry.
That's decentralization.
BTG isn't competing with Bitcoin or other alts - BTG is building something useful and accessible to developers and people, and the ecosystem is improving steadily. In the future, different kinds of coins and miners can co-exist and interoperate more than ever before, and the questions will be: what are people mining? Buying? Holding? Transacting? BTG is steadily building towards allowing more people to do all of those, while making sure it's accessible to everyone - not just the rich and connected.