TL;DR:

Looking Back: This is where we directly respond to the goals we created last set in our previous Learnings article.

• Legends: Legends were a big conversation of our last Learnings article. They won’t be returning any set soon, but the design space remains of interest to us. Until we can find a system of pre-game selections that doesn’t come with the negative impact of reducing variance a wide amount, we’re not rushing to jump back into the space.
• Region Portals: Portals are about as TFT-y as it gets. At this point, we believe that Portals are like Augments, and are going to stay as a permanent part of the game.
• Power Bloat: We had the goal of lowering power slightly (namely three star, four and five-costs) after Runeterra Reforged and we did this through adjusting a few systems. We did this with Level 10 and bag size changes that are here to stay (for now).
• Skins and Accessibility: Base skins are really easy to read, and Runeterra is just a fun place to come home to. We don’t have anything to announce right now, but I can confirm that at least one set in the next two years will take us back to Runeterra!
• Exciting and Unique Low Cost Units: We’re still continuing in the direction of making low-cost champions unique, fun, and having their own fantasy to explore for our reroll enjoyers and flex players alike.
• Set Complexity and Balance: Set complexity makes balance tough. The more moving parts, and possible combinations exist, the more things you have to account for for balancing. Despite Remix Rumble being our most complex set to date, we did pretty well on the balance side of things with our only B-patch (mid-patch update) being a planned one over the Riot-wide Winter Break. Hopefully this trend can continue.

Looking Forward: This is where we set out a list of goals to improve upon for the next set!

• Game to Game Variance and Loot Distribution: Moving forward, we want the item and loot drop system to remain one of game to game variance, NOT player to player variance. Editor’s note: Read this section for a systems change spoiler!
• Mechanics & Overall Complexity: When examining our past ten sets, the most successful set mechanics are the ones that add variability and complexity that keeps each game feeling totally fresh, but with Portals and Augments as evergreen mechanics, we can experiment with set mechanics to achieve thematic goals, or just make a set feel unique.
• Champion Skill Intuitiveness: So in the future, we’re going to be paying extra close attention to our champions and trying to make them as intuitive as possible from a glance, with the extra details to be discovered on the tooltip.
• Economy Traits: One of our big challenges moving forward is that we have to keep reinventing this space as players solve our economy traits each set. This won’t be easy, trying to do it 3 times a year, but we are committed to it!
• High Moments & Prismatic Traits: We’re taking steps to make sure Prismatic Traits and other High Moments are hard to hit, and worth hitting.
• Item Uniqueness & Artifacts: Artifacts allow us to do very cool things that our core item system does not allow for…so we’re adding over 20 more new Artifacts that’ll come at some point DURING our next set.

Full article https://teamfighttactics.leagueoflegends.com/en-us/news/dev/dev-tft-runeterra-reforged-learnings/