It freely allows you to construct decks with any card (you can even make your own). Can do multiplayer (client/server), multiple different modes like planechase, archenemy, commander, etc. You can download other people's decks from the internet and play against the AI. The AI is alright for the majority of decks. There is even an '97 styled adventure mode that is pretty true to the original.
Slay the Spire is great for sure, but for this I'm asking about deckbuilders with the style of deck-vs.-deck like Magic. In other words opponents use cards as well, and you may unlock the cards they use yourself as you play.
Monster Train is close to this, but has its own spin leaning into roguelike design as well.
legends of runeterra pvp is on maintenance mode but it's still playable. i don't play it anymore but at its peak i strongly preferred it (as a homebrewer / ranked mode grinder) to the two in your title.
Long ago there was elements the game. It was an excellently designed game that had a lot of potential and was fun to play. Lots of deck building options. They came out with like one expansion that showed more potential but it stopped being worked on. If I recall later into the game the grind to collect rares was really annoying.
Unfortunately the site's completely dead now but after some googling there seems to be a fan base trying to revive it, idk how that's going but I'm going to check it out.
It really was amazing. Such a unique design for a card game nothing is quite like it. I checked out the most popular app of it and they recreated the game! Unfortunately, it's an exact recreation which means the grind of the old game is still in place which is really disappointing. I'm convinced it would be a popular game if it was closer to a modern online TCG and I didn't have to spend hours to grind out new cards for a deck.
There are several single player Magic The Gathering games, but I think only the first one from 1997 gives you full deck building flexibility. The sequels just have a bundle of pre-builts to choose from.
The 1997 version has one-on-one Constructed and Booster Draft modes, in addition to a roguelike-RPG game mode.
If you liked shandalar, you should check out forge like someone suggested in another comment on this thread: https://github.com/Card-Forge/forge
It's a free, fan made, FOSS game that implements even the newest cards coming out, and it has a shandalar inspired adventure mode for you to play, where you can defeat all color wizards, collect cards, play drafts against AI and such, it is a great time.
Okay this is going a little bit left field, but Esports Godfather was one of my surprise hit releases of last year, and its currently on sale as well.
It's not purely a card game as there are other parts to the gameplay, but at its core is effectively a deck builder, and one where you play out games 1-on-1 like you desire.
It plays way better than you might think at first, trust me.