Legends of Runeterra is the obvious step up from Hearthstone. It offers almost everything that is good about Hearthstone and much more (if you’re like me anyway).
Gwent is another but it plays nothing like Hearthstone, it doesn’t have the same game feel but, from what I understand, it’s very technical. Both LoR and Gwent are very generous F2P games.
Hearthstone is inherently one dimensional for most players until you reach specific parts of a matchup. The first 4 or so turns generally play out themselves, and unless you know the matchups inside out spotting other lines of play is generally not going to happen during those turns. The complexities of the game are really only prevalent at the higher levels.
With LoR, as big decisions get made earlier in the game (and more of them), often around turns 2-3, it relies far less on RNG and more on the lines of play each player has chosen, which means games between middling player (Platinum to Diamond in HS) are going to far more often feel like they were won due to player choice. There are clear areas of improvement to make when trying out new decks, where as in HS these improvements taper off after you’ve reached a certain level, and can really only be improved further by becoming a very good player if that makes sense.
Deck building os also fun and varied as decks are often reliant on Champions (of which only 6 are allowed in a deck), so middling players can easily identify synergies and test out cool things which aren’t seen on the meta.
There’s far more to get into tbh, I’d suggest you just try it, give it atleast a week or so to adjust, and I would be surprised if you don’t come to enjoy it.
I second pretty much all Kinkyjohnfowler said and want to add some additional points regarding the OP's questions.
1. Diversity: LoR is also a rather combat centric game, but the focus is more on the interactions of both players' boards during each turn. The field of decks is very diverse in terms of how spell- or minion-heavy they are, but spells HAVE to be a lot more monitored/restricted for balance's sake in LoR, due to the interactivity of the game, their "higher speed" of impact on the combat (due to many spells being somewhat 'faster' acting than minions) AND, in the case of spells that can target face, the lower hitpoint threshold of each player's "face" (called Nexus in LoR). In HS you have to balance cards only around one player's turn at a time, due to it basically being alternating turns of solitaire . In LoR cards have to be balanced for the situation, that both players can highly interact with each other during each player's attack turn, hence there are different cast speeds of spells and so on. Board clears are a little overprized for their efficiency in LoR, which means that there is no real attrition deck. There are a lot of reactive decks, but they all have a finisher in some way. In LoR it is very possible to be successful with experimental decks, cause the game offers a lot more 'hidden' synergistic possibilities (reason for that is, again, its interactivity during combat) over HS, where synergies are more obvious and therefore deckbuilding is easier.
The combat system is very complex and involves a lot of possibilities for mindgames, bluffs and so on. For example it is possible to kinda 'burn' the opponents mana, by 'aggressive passing' of turns, offering him to either buy into your bluff or forfeit his mana and right to attack for that turn, returning the attack token to you in the process. While in HS curving out is the best thing to do in 90% of cases, in LoR you will see hardcore curving out only on lower ranks, cause simply always using all your mana at once will leave you prone to your opponents reaction, which he gets to make right after your play. Sometimes it is right to just curve out, but in higher leagues the art of passing turns becomes a lot more important. So, why is that, one may ask. This has to do with the fact that the spell (and also some units) system of LoR very often rewards the re-acting player over the one who made the pro-active move. This has to do with 2 facts: 1. simplified spoken (its too complicated to explain properly, but you will understand it when u play it) whenever a player takes an action, the other player gets to make an action thereafter and 2. Fast and Slow Spells (which are 2 of the 3 spell types) resolve in backward order of how they were cast. This means the spell of same speed that was cast by the re-acting player will resolve first. Which is basically the exact opposite of solitary based combat like in HS, cause there you can bluff so much (and also have to). Games where you have the mana to finish the opponent, but it is better to hold it for an upcoming turn OR for the case where he ventures out and makes a risky play, leaving him with either not enough mana to react to your coup-de-grace or simply beating him by the fact that your spell resolves first, are very common in higher leagues.
2. How f2p friendly is it? Verrrry. :) I completed the whole collection after 3-4 months of play (but i played quite a lot). It is so darn generous. You wont need the whole collection tho, to build all strong decks. There are top masters accounts who have only 70% of the collection. And if you want to buy cards it is a lot more generous than HS as well.
3. Community/popularity: For me, this is the only field, where HS is clearly better than LoR: LoR's chat system is a bit clunky and harder accessible than in HS, where you can chat and receive messages easily within any screen. This leads to players often reacting later to chat messages than for example in HS - simply because they weren't aware, somebody sent them a msg. Also, in LoR you can't add your most recent opponent in any play mode but one. BUT there is a trick if you want to do it anyway, that works about 70% of time for me. I added big parts of my friend list of now about 70 players this way. The final thing where LoR is worse than HS is that you can't observe your friends' ladder games. You can observe their tournament games tho, cause those are played in Friendly Challenge mode. You can also observe their casual games and expedition (same as arena in HS) games. For LoR in terms of community building i highly suggest that you visit the discord servers dedicated to the game.
So, maybe this has tickled your interest. I am playing on EU in LoR, have played in masters (highest league) last season and am maybe doing again this season, if i overcome my ladder laziness. ;) If you decide to try the game i can supply you with answers to any question you have about it and will give you the 'fastest route' to getting the maximum amount of XP per day and therefore a full collection eventually. My in game tag is: a3b2c1 #5685
Edit: LoR is a lot more transparent than HS in terms of other players on the ladder (be it master players, whose profiles you can freely see in the game or players from your friend list). So there are quite some stats of other players to look at and also you can see their most recently played decklist, which is also very interesting.