The problem with adding leaders is all the time and resources required to "fix" this optional feature while ignoring issues many like me don't care if it gets "fixed". I have much bigger issues with the design than Machiavelli of the Mississippians. (Which the game already automatically doesn't allow - Machiavelli will already paired be paired with Greece or Rome but never Mississippi unless you specifically choose that.) There are some leaders like Ashoka you could connect with India but someone would probably have a problem with even that. This is an exercise that, because the game wasn't originally designed to support that concept, it could just end up with Firaxis chasing their tail. I would vote against them dedicating resources to making 15-20 new leaders instead of all that art and design being applied instead to legacies, UI updates, the entire concept of 1 civ through the ages, new updates like the naval stuff, etc being completed faster. Things that would improved the game for everyone, since there is plenty of those options on the table.
Those leaders were designed to be a figurehead of a singular Civ's overall design. They were meant to slightly compliment that civ's abilities at best so it is not the same as Civ 7's leaders were designed to be a stand alone mechanic that is more isolated and therefore stronger relating to a pool of civs. Civ 7's leaders are actually more like entire civs in 5 or 6 as they are meant to span the entire game, not just 1 age. Asking for 15-20 more leaders to be balanced into the game is a big ask and then what if those arguing decide they don't like how Firaxis matched them up and don't buy for that reason. I completely understand people disapproving of the direction of Civ 7's design but I don't understand people wanting Civ VI part 2. I totally understand that Civ 7's design is so drastically different that I think you either have to accept some concessions or just walk away from Civ 7. Firaxis agreeing to trying to make a classic mode is already a big gesture, "the artist is painting the picture differently (in a way he didn't want to) due to fan outcry". To then start requiring very specific demands, if I were the artist I would just hand you the brush metaphorically. Go make a civ game yourself or mod in the leaders. That is not me telling you that - that is what I would say in that hypothetical scenario. I want to point out that this large detour in design is a concession by Firaxis, and yes, it is probably due to sales and profitability. But I think the community should at least wait and see what the classic mode looks like before we start demanding an additional 15-20 leaders for free in a design where leaders are not just an art asset tacked on as they mostly have been in the past.