Currently, there is a healthy gameplay trade-off between growth rate if citizens are on a core world, and better productivity if they are used to colonise / crew a star-base. Citizen management is currently in a pretty simple yet deep place.
Anything that increases the stay at home value of a citizen will make all the stuff that affects citizens and growth already suddenly becomes more potent, and I think it's in quite a good spot at the moment. If a placed citizen didn't contribute to growth rate, that'd deal with that issue, but doesn't make much Ludo narrative sense, and impacts synthetics weirdly. Something to think about.
If you kept tile improvements, then adding citizens to tiles like this speeds up a planet's development for non-synthetics (as they go where a tile improvement would normally go, so less time to full development), and means you'll have to go through and think about racial growth rates, and possibly racial stats, not to mention any tile improvement suddenly has to be thought of relative to citizen bonuses. If citizen bonuses are too high, the game becomes about food and pop limit, whereas if they are too low, it doesn't change anything. In the sweet spot, you get more complexity for a limited amount of extra depth that is very very dependant upon your racial choice (and interacts with the ideology that gives you 3 random citizens, and any citizen granting event). This "sweet spot" has to be in the sweet spot for every race and most civilisation options, or you've only added depth for some races and civs at the cost of a lot of effort.
Weirdly, if you just transport a load of citizens to a new planet with this "citizens instead of tiles" philosophy, the planet would spring up super quick - I dislike that, as a strong core getting weaker the further out you go is core to war-time border moves and resiliency. If you can just turn worlds pronto into super fortresses for the non-sunk-cost of using transports, I'm not sure that's a good direction. Although being able to up sticks from your average homeworld to all live on the super precursor world you just found would be pretty funny.
From a gameplay depth view, if citizen placement interacted with the adjacency and/or the tile bonuses, you could use citizens to compensate for / accentuate the benefits of different planetary layouts, meaning that they'd interact with inputs in a way that added more depth.
I'm generally against this for galciv4, as it feels derivative game identity-wise, and doesn't add to what I am enthused about. Much was made of the combat mechanics meaning you don't just build the biggest ships, but it's hard to see from tooltips and battle reports why this would be the case. Also, the way experience for ships works means that surviving ships get hugely stronger if you can afford to upgrade them with the good upgrades, to the point nothing else can challenge them. Also, being able to refit ships into a same-hull-variety different design would be nice.