The problem here is mechanics vs reality.I've been thinking about this a bit more, mainly in how do you balance tapping into a land and I figured that we already have attunement which could be used for this by having attuning to a land take up one of your attunement slots. If you're a planeswalker, however, you can tap into additional lands above and beyond what non-planeswalkers can tap into (planeswalking might be as simple as a supernatural gift).
I was halfway tempted to adapt the birthright system and have more powerful lands provide a bonus spell slot that can be used for a spell of a specific colour but that gets a little needlessly complicated, you'd probably want to be playing Birthright the Gathering if you were going to use that system. So I think I'd have basic lands allow the caster to cast spells of a related colour as if with a higher level spell slot (though this doesn't translate perfectly since many spells don't have a higher level effect) while unique lands have focused the energy to grant some specific bonuses such as increased damage with red spells or buffed beast summons with green spells as well as some once/day innate spells like I've already mentioned in my previous post.
One of the early books had 'lands' represented by tokens taken from places people had visited, representing what they could draw on. You could kill someone and take their tokens, and thus access to their Lands.
In game terms, you could simply do this, with more valuable tokens linking to lands rarer/more expensive, and you limit the number of lands by a combination of Stats and Levels.
This in turn naturally limits the amount of mana you can draw on, and the cap the maximum you can draw on at one time. Artifacts that increase mana, creatures, etc, increase this cap.
As for 'playing land', it would have to start with how many lands you can bring into play, and how fast you can do so. It means that ALL fights would start with small spells, so they can't be avoided, and move rapidly to larger ones the longer the battle goes. This is actually thematic, as the winds of magic get stronger as conflicts brew... but it means casters start weak and finish strong, which can be lethal when facing monsters.
You also have to have iron rules on when the 'lands' vanish, because you certainly can't leave access to all lands all the time... or maybe you can?
Maybe that instant of two mages conflicting severs land connections, and you have to reset your magic, and OUTSIDE of conflict, given time, you have contact with all your lands, if they take a little longer to cast.
Then start connecting lands one at a time. Now, randomizing or putting an order to that would probably require you to actually have the cards to shuffle and play... but that's a different issue.