Adding more Champions to TFT is a dangerous road with the Game's current RNG
Even in its current phase, what i consider TFT's most frustrating thing after the ITEM RNG is the Champion RNG. I have had games lost because i couldnt get a crucial unit to level 2, even after literally spending 60+ Gold, throwing away everything because i needed literally 1 more of that unit and it didnt show up in more than 30 rerolls.
Adding in more champions further decreases the already potentially low RNG chance of getting the one you want, unless something else is done.
If Riot plans on adding more Champs and origins like Twisted Fate and who knows what else, either A. We need 6 slots to choose from in the champion rolls or B. RNG needs to be tweaked to have higher chances for champs you have. Say like 5%+ chance for a champ you have 1 unit off, and 10%+ for champs you have 2 units off waiting to become a tier 2.
Obviously this would have to be carefully implemented as well to avoid many people getting multiple Tier 3s, but i think its more frustrating not to be able to at least get that Tier 2 you really need to round out your comp