I'm a little late with this, but life intervenes! So I got to check out several of the presentations from Sitecore Symposium, and I'm sure I'll get to looking at more of them on-demand as time permits. I believe the on-demand sessions will be available until the end of the year. Here's a few thoughts I have:
You probably noticed a few sessions on containers, with Docker and Kubernetes. Everything I've heard says that as of Sitecore 10.2 (probably about this time next year), Sitecore will be moving away from the PaaS model on Azure with app services to containers. It's not 100% containers on Azure, unless you want it to be...you could have it all encapsulated but Azure SQL is still very much available, and you can still run Solr via SolrCloud (like with Searchstax). For performance reasons, you'll likely want to have those systems be where they specialize in production, but in QA you could have it all together.
In recent days, I've seen folks on Twitter wondering about the cost-effectiveness of containers, performance monitoring, etc. The consensus is there's a big learning curve, which luckily we have a year to get ready for. One thing that's a frustration to me is Sitecore packages. You can set permissions on the container to allow the normal installation of a Sitecore package, but when that container spins down, the package vanishes. You'll notice this in some of the standard Sitecore containers, where there's a setup for Powershell Extensions, SXA, etc. Basically, if you want a Sitecore package to consistently install, you'll need to create a container for it to connect it up. Since this is a one-time operation that can be reused, but there's the learning curve of doing it. Alternatively, I suppose, you could create a Helix project for the package instead, like just taking the source from Github or wherever, and have the code and items install like any other project, but that leaves a lot of potential maintenance in other ways.
Anyway, as noted, we have about a year to get ready for this, and I'm sure this stuff will streamline more as we go along, whether direct from Sitecore or through the community.
There was a walkthrough session on the new Sitecore 10 serialization setup, and the Visual Studio plugin that supplements it offered by Hedgehog (now owned by Sitecore). It's all JSON based, and you can follow the documentation to set it up, but the VS plugin will help you manage that as well, like TDS would do; the difference, of course, is that the new plugin doesn't have its own set of projects to double-plus the size of your solution. Think of it as the offspring of TDS and Unicorn. That said, it doesn't have feature-parity yet with Unicorn, and it's not backwards-compatible beyond Sitecore 10, so there may be some time taken to adopt it. From the Helix standpoint, Unicorn still seems to be the "gold standard" and it's not shutting down anytime soon, talking to folks that help maintain it.
One of the sessions I was most looking forward to was Shelley Benhoff's Helix startup session. If you haven't checked out her courses on Pluralsight, I highly suggest them. She went into using Yeoman to do Helix project setup, as well as Anders Laub's Visual Studio plugin module (which has finally been updated on the marketplace to match what's in the course). Personally I like Anders' model, with the flexibility of modifying the templates to your liking...maybe I'm not as familiar with the Yeoman model and how to modify it. But these kinds of toolings are important, especially as we get more complex in the solutions we put together and the need for rapid development.
As I get into some of the other sessions and review them, I'll try to add further follow-up. The .NET Core renderings talk is one to revisit for sure, another of the "new hotness" items coming along that's got its starting traction, but isn't fully realized throughout the product yet. When these things come out, we definitely need to take heed as developers and prepare, but don't lose sight of the "now" for our clients, especially since they're not going to be on the edge of the new developments. For myself, I've got an upgrade early next year that will likely be my first v10 deal, and all of the bits I need now support it. As time goes on, we'll see about starting to adapt in the newer techniques. But never forget where the product's come from! (Except XSLT renderings...maybe we can forget those...)