Great conference, congrats to the organiseers!
Some ideas I had on the way back from NIME:
Free remote participation?
It seems there were not many people people participating ‘remotely’, but I assume each additional person would have been at no additional cost to the conference. This suggests to me that the price point might have been too high, and on the basis of NIME’s open access principles, personally I feel it should have been free to watch the stream, with the charge only have been for those presenting works. I would be surprised if anyone paid the £100 - £250 to join online without being a presenter (and with institutional support).
Where I interacted with remote and local in the discord I had a really rich and thoughtful exchange, and this would have been much better with more people here.
Again I feel open access principles should apply to presentations as well as reading papers.
Having free online registration could also greatly improve the diversity of the conference.
Travel and non-travel
I understand a hub-based model like previous ICMPC/ESCOM has been considered but not implemented due to the organisational demands.
A relatively easy to implement (for the organisers) and I think uncontroversial mitigation is to encourage satellite events. These come in two categories: a) watch parties where people can come together to watch the stream, and potentially host speakers/performances to be streamed back to the conference b) events in (where geography allows) nearby cities/countries so that people who are flying in can take a longer trip, touring, performing, and generally making the most of opportunities for cultural exchange @federicovisi did the former in Berlin for our alpaca (algorithmic patterns in the creative arts) festival conference and it ended up being more like a hub, hosting nice local audience with talks including the keynote. I learned a lot from this experience but it was definitely something to build on. Most of the presentations for this part of the conference were online and I think it was a great experience for the presenters to hear the applause from Berlin. 
There were also watch parties in Linz and Barcelona ICLC does more of the latter and participants have a great time spending a couple of weeks travelling locally and hanging out a week before/after the conference itself.
Hopefully this reduces environmental cost where people are doing several trips in one, but in any case will be getting a lot more out of the experience thereby making that cost more justifiable.. and just being a lot of fun for the people involved!
Workshops as mini-conferences
Relatedly, as the topic of NIME broadens, I think the workshop category should be expanded and ‘mini conference’ proposals encouraged, with enough time for light touch peer review processes of their own. These can be independently organised, taking load of the main conference, and potentially helping it return to its single-track roots.
More than once I’ve been really frustrated by trying to host mini conference-style workshops at larger conferences (this might have included NIME, I can’t remember offhand). Firstly, they have demanded participants pay for full registration even if they only want to attend the workshop. Secondly, they have required that the workshop uses the main conference system for remote participation, for unclear reasons.
I think it would be great if two or three days could be given to workshops, maybe allowing them to be full rather than half days, and allowing them to be independently organised.
Live chat/discussion
Discord is a proprietary platform, I do enjoy using it but longer term it will probably get worse!
The discourse forum is really great, and now has an inbuilt chat system that could be enabled. I’m not sure the discourse chat is up to the task of hosting q&a and discussion yet, but might be worth a look.