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I am not sure "replace the Heroes of the Lance with your own PCs" really gets us anything particularly interesting. We all know what's at stake, the major issues and plot points that'd have to be hit. I can just read the books. I think we'd be better off with, "While the Heroes of the Lance are in motion, what else is going on?"
Yea that might only be interesting for people who do not know the story and did not read the books or absolute hardcore RP enthusiasts who in that case, might want to recreate the thing using the original characters.
Your suggestion sounds a lot like how the LOTRO MMORPG works, you are above below in front and behind the protagonists to fix things without altering the metaplot.
Tbh that metaplot is one of the biggest strengths and weakness of DL at the same time. Back then in 2e/1e times we did already note this. The gold box computer games which were DL themed were - although pure tactical combat, and meager graphics compared to todays stuff - absolute fun and challenging to play, but you rarely interacted with the big boys.
Other computer games around that theme used the heroes steered by the player.
The most impressing thing was Dragonstrike, a D&D based dragon flight/combat simulator.
There you could reenact what a Solamnic knights real task was, riding a dragon armed with a dragonlance and controlling the dragons breath weapon, also with the lance.
Not many other products had this level of epic I can tell you that.
But getting that feeling to the table is difficult, when at the same time you got to push it in the right direction so that possible deviations to the metaplot by your PCs do not lead to absurd situations in which you only can wing it from one improvisation to the next and maybe miss the intended target anyway.
That is helluva difficult for a DM, considering 5e freedoms which were not available in 2e. I have some good ideas and read about good ideas how to do the mechanical stuff and some of the rules fluff of the setting in 5e, but other than your suggestion on how to handle the metaplot or others similar ideas, I do not see how this can be done in a simple way.