something to spend your money in

Very rare? Ran into 3 eevees in a row on my way to venisi, but then again I never had any problem finding a particular pokemon. But seriously just get a ditto and use the free eevee you get as breeding stock.

Eevee is rare in the wild of other games, not this one.
Besides, we have the casino already. You get Tracton before Victory Road

Personally I did have an idea for a post game area where we can buy pokemon. It just has to be any pokemon you've seen before that is non legendary. Price goes up based on stats (IV, EV), level, shiny or not, moveset (eggmoves, tms), and server values of rarity.
The server values will modify the value of a pokemon based on data it receives from games. The more that people bought a chyinmunk for example the higher the price rises. If people were buying Chyinmunk that had quick attack and endure (or whichever was the rattata strategy), that specific Chyinmunk build would go up in price. That means if you generated a lv 100 chyinmunk with the moves specified it would cost more than without those moves. If people had never bought a chyinmunk with those moves, or not during that week (a cooldown effect) the Chyinmunk won't cost as much.
Additionally, this means that people could buy low level pokemon at a bargain, but they could also generate competitive worthy pokemon (no raising needed) just out of the box with enough moola.
This would be a black market in essense, or a Pokezoo. Players could additionally capture pokemon to sell on the black market. Instant cash, but because this isn't legal you get money relative to rarity.
Metalynx with Frenzy Plant at lv 50 will take in more than a Chyinmunk with Cut at lv 50.

The rarity values could additionally take data from Wonder Trade and GTS.

However, this is a pain in the ass probably to code even when using the existing code structure JV is likely making for the Wonder Trade and GTS, AND I have no idea what post game has planned so this is again another suggestion that is likely a waste of time.

(I also had an idea for gambling online, pvp poker, hanafuda, etc, but again, that requires a bit of effort)

Let's just drop money spending.