We did something, it is Segwit. I think it is tomorrow when it should activate.
#1) I don't know about this. Some cryptos do this to get rid of mining pools. There might be some drawbacks, but as other proposals it would definitively require a hard-fork and we know that this can take a lot of time.
#2) I don't think that differences between blocks have as much to do with daily changes in hashpower as just with luck. I don't think that this would help at all. This readjustments are representing the change in network's hashrate and I don't think that there is a significant change in hashrate on a daily basis.
#3) Woah there, this sounds like centralization to me. Nodes are represented by IP addresses and that means that whoever can get most IP addresses has that 1%. ISPs and therefore the governments would love this.
#4) Any suggestions on how to do this? I think that reducing block time would just increase the rate at which disk is accessed.
#1) There are a ton of coins that have low block times. They do it to make fast confirmations. Who wants to wait 50 minutes for a confirmation? Or even 10?
#2) Faster difficulty readjustment periods prevent nefarious actors from pushing difficulty up and leaving the network. This creates longer transaction times as time between blocks would be higher than it should be. This would have been helpful during the BCH split. Difficulty would have readjusted in a little over 12 hours.
#3) No, The way DASH did it was to require the people to keep coins in the wallet. Anyone can setup a full node, but the number of master nodes, aka, the number of people who are able to get a payment for their node would be limited. It guarantees a minimum number of full nodes on the network. As of right now, I don't know anyone who runs a full node and your concern about nefarious actors setting up a large number of nodes is true even without master nodes. I could go setup enough master nodes just by myself, right now, to cause problems on the bitcoin network. I'm honestly surprised no one has done this yet. Master nodes will be necessary the larger bitcoin grows. It will come to a point where only exchanges and pool servers run them.
#4) UTXO commitments ? There are some not so insignificant issues with this on bitcoin though.