What you describe is a fairly common "promo" incentive, once upon a time you might get it in the version bought at a big event or that sort of thing, these days it'd be for Kickstarter backers.

Yes, you can make small customisations that don't significantly change gameplay, for example Viticulture is a game about owning a small vineyard and to add variation you get a "Momma" and "Papa" for your vineyard which tweak how much money or starting equipment you get plus they have names and little illustrations to give them personality. Maybe Papa Phil gives you slightly less money than Papa Eric but a piece of equipment that's worth the difference if you need it - having Papa Phil saves you taking an action buying the equipment with money, but if you don't use that equipment at first you're still short of the cost whereas someone playing Eric can spend it how they wish. Clearly having one or two extra Mamas or Papas as a promo isn't game breaking (so long as the designer doesn't add Papa Scrooge who has twice as much money as everybody else put together or suchlike). In fact Viticulture went with event cards for Promos, which is a bit trickier to balance, but seems to have worked OK in practice.

But often the correct way to design games is to simplify until it's good, not add more and more until playtesters would rather say it's great than risk being asked to waste another six hours on one more test session. Promo features fight that. Would Viticulture work fine with no Mamas and Papas? Yes it would. Would Can't Stop be better with fancy cosmetic player pieces instead of the typical generic ones? Er, no, not really. So such promotional features aren't very good for the hobby as a whole.

Piece organizers, additional players expansion are also good stretch goals.

Cosmetics would be where I would put the stretch goals at (like upgrading the coins to metal coins (remembering to adjust the shipping cost accordingly), upgrading the player tokens, etc), or very expensive t-shirts (as those print runs can also get complicated fast), or stickers or other "simple" swag.

Disclaimer: Have not run or funded a board game Kickstarter, only heard a podcast about one (https://www.idlethumbs.net/3ma/episodes/threes-a-crowdsourci...)

What would be an interesting stretch goal is someone promising to take the extra money to spend more time thinking hard on how to remove and simplify mechanics.