I think it would be easier if at every holiday, You would be able to get a holiday egg, which will give you holiday Pokémon. These Pokémon, depending on what season it is, will change how they look. So let's say, at Christmas, you got a holiday egg, and you got a holiday squirtle. It's thanksgiving, so it looks like the thanksgiving squrtle. When Christmas comes, it will look like a Christmas squrtle. It will (hopefully) solve the issue of people trying to collect every holiday pokemon possible.
I get what your trying to convey, but that would be pretty difficult to do and it sorta removes some of the purpose to "catch em all"
Not all Pokemon that have Halloween eggs have thanksgiving eggs and more Pokemon will be added each egg so I don' think it' fair to put the strain on the modeler since there is only one modeler