Read the article here, but I did my best summarizing the most important points.
https://www.bungie.net/7/en/News/Article/lightfall-year-ahead
Four primary goals for the year of Lightfall
Expand players’ imaginations
Bring challenge back to Destiny
Enrich our content
Connect our Guardians
Season 20: Season of Defiance
Season 21: Season of the Deep
Seasons
Umbral Engrams going away, replaced with Seasonal Engrams now stored on-vendor. Focusing will now just cost an engram + glimmer
Seasonal currencies getting removed/reduced, you now get a flat key for the "final chest". Chest is a true upgrade on rewards, no longer necessary for regular loot. These "keys" won't drop as often, so the regular activity will drop more loot itself.
The keys can drop from the activity, so you don't have to play non-seasonal stuff to engage with the season-itself anymore.
Fewer vendor upgrades, more meaningful changes, clearly descriptions. Less time reading, more time playing.
Season 21 will not have a 3x7 vendor upgrade grid.
Lightfall Seasons will be pushed to be more creative and unique like Shattered Realm and Chosen's original reveal of Battlegrounds.
Guardian Ranks will be replace the "Season Pass levels" nameplate shown next to players in Lightfall
Crafting
Fewer weapons will be craftable in following seasons
Non-Craftable weapons (starting with Raid Adepts in Season 21, slowing extending to all others) will eventually be able to be "enhanced", granting Enhanced Perks + Masterwork Bonuses + Mementos + Etc.
Any red border that drops will only be for weapons that have a pattern. If it has a red border, you instantly know it's important and a valued drop.
In Season 21, you will be able to target any craftable weapon with a guaranteed deepsight drop
PvE Combat and Challenge
To increase combat difficulty, Bungie is taking a two-prong approach to combat
1st Approach: Decrease ability uptime but maintain full ability potency. Reduce strength of resilience and some combat style mods.
2nd Approach: Increase combatant difficulty by introducing power-cap scaling throughout the game. Right now it'll just be on the seasonal activity + vanguard ops + Neomuna Patrol.
More experiments on capped light levels/fixed difficulty to come. "The Big Change" will come in Final Shape.
Power Levels will still be a thing in Lightfall. However, in Season 21, the power cap won't even be raised at all.
Crucible
Reintroducing Countdown (Search and Destroy) and also a new variant Countdown Rush, where you must arm/defuse both of the bombs on across the map
Checkmate Control: Highly customized sandbox mode. Seems to be very personal-skill focused based on smarts and gunskill.
Weapon damage, ability uptime, and even ammo are all adjusted in [this] new mode. Will reward players who use their smarts and their skills. So, if the only way the enemy has been able to shut you down in the past is a solo Blade Barrage, they might be in trouble.
Meltdown (Close/Mid Range Martian Map from Year 1) Returning in Season 21
Brand New Vex Network Map in Season 22
Citadel (Medium/Long Range Dreaming City map from Year 2) Returning in Season 23
Matchmaking adjustments will continue. Dynamic Skill Ranges and Team-Balancing focus for more equal skill teammates and high-connection lobbies.
PvP Reward Restructuring. Both Crucible and Trials of Osiris being reworked. Competitive ranking improvements/speed up
Exotic Mission Rotator
Starting in Season 22: Presage, Vox Obscura, and Seraph's Shield all being added on a weekly rotation. More classic missions to be added "back into the fold" (Whisper/Zero Hour?).
Strike Rework
Lake of Shadows and Arms Dealer both being reworked, refreshed, and updated to match current strike quality levels (think Lightblade and Proving Grounds).
Inverted Spire and Exodus Crash being soft-sunset (still accessible from director) until they will be reworked next.
Battlegrounds being moved into the Nightfall playlist. Mars Heist Battleground being the first focus (Grandmaster Escalation Protocol? Better gear up...)
4 out of the 6 Nightfalls in Rotation for Season 20 will be either refreshed or newly featured
Ritual Content Refocus
Much later in the year (probably Season 22-23?), more content and rewards will be pushed towards the ritual playlists following these updates
Lost Sectors will no longer be the source of brand new initial drops of seasonal exotic armor, instead moving to the core ritual playlists
No longer need all three ritual ornaments for seasonal challenges
Slow burn until Final Shape, taking the entire year to refresh the ritual playlists and refresh our core activities
Social Connections
Commendations are an icebreaker for meeting new guardians and rewarding good effort. Trials Teammates or Raid Leaders/Coordinators.
Guardian Ranks aim to show the best of the best in the community, people you can trust
Text Chat moving to Opt-Out instead of Opt-In
Still will be able to leave individual chats, and they plan to improve moderation, filtering, and even Speech-To-Text options.
Fireteam Finder
Being delayed until Season 23's dungeon, regrettably not with Season 22's raid.
Truly fleshing this out to be a first-class system with keywords, tags, control over fireteam members, automatic joining or allowing restricted vetting processes to deny/approve individual people who want to join
Queue anywhere in the game for any activity