Adding more champions frequently is a horrible idea unless they change the way champions are offered in game.
Currently there are:
- 13 Tier 1 with 39 of each champion
- 13 Tier 2 with 26 of each champion
- 13 Tier 3 with 18 of each champion
- 10 Tier 4 witih 13 of each champion
- 8 Tier 5 with 10 of each champion
Ignoring the scaling odds per player level and other player pools, here's what you should expect. Assuming you are offered only T1 champions, the first one gives you a 7.69% (39/507) chance at getting a specific one. A second T1 champion gives you a 7.51% (38/506) chance at getting a duplicate. A 3rd T1 champion gives you a 7.33% (37/505) chance at getting another duplicate. Your odds for getting more duplicates shrink each time you get one.
As you level, your chances for even getting the tier you want change in favor of higher tier champions. At level 6, you only have a 25% chance of getting a T1 at all. So hypothetically, if you are level 6 in a sandboxed environment with no opponents and have bought zero T1 champions, your odds of getting a specific T1 champion are 1.92% (39/507 x 0.25).
Adding one champion
If just one more champion is added to the Tier 1 pool for a total of 14, the odds are now shifted downwards for getting specific T1 champions. Odds of the first are 7.14% (39/546). Odds of the first duplicate are 6.97% (38/545). Odds of the second duplicate are 6.80% (37/544).
For the hypothetical level 6 with a full T1 pool, that's 1.79%, 1.74%, and 1.70% chances of getting the duplicates for subsequent single slot offerings.
If they keep adding more champions, the odds of getting the specific champions you want will continuously drop. If they want to continue adding champions, they need to start restricting available champions to keep the probabilities usable. They already know it's rough with the existing probabilities, which is why Neeko's Help was added to the game. Maybe have a max of 60 champions available at any time, with a rotating champion pool per patch cycle. This is still problematic because every champion contributes to multiple set buffs, so leaving any champions out of the game automatically makes reaching those synergy requirements harder.