I don't believe IVs can be adjusted. It isn't mentioned in the tutorial text and I suspect that they just forced the max value on everything. EVs can be adjusted at any time and they seem to have simplified it.
There is a campaign mode with two paths. Held items, natures, and specific pokemon are gated behind different progression points. The text mentioning the pokemon did not outright say legendary or mythical, meaning it may be possible that they will gate high usage or more popular pokemon behind it. It is made clear that you will still have to have campaign mode gated pokemon in pokemon home to use them. Campaign mode battles sometimes allow you to bring your own team, and sometimes you're given a preset team. These battles can be singles, doubles, triples, rotations, inverse, type exclusive, raids and possibly more. I do not know how difficult the battles are, but you have only five daily attempts before you will have to spend one of the two in-game currencies for more. You cannot switch between paths freely. One path must be finished first. The campaign mode paths will have new additions with each new competitive season that is added.
There will be seasonal events. They will have extra cosmetics for your character, pokeballs, and entrance effects. These require you to spend a daily attempt for each progression point like the campaign paths.
Victory Points are a free currency earned through tournaments and other events. Challenger Points are a paid currency.
Players will be able to set up rental teams with two privacy modes either making the team public to all or friends only. These teams are restricted to friendly matches. They cannot be used in tournaments, seasonal events, or campaign mode battles that allow selecting your own pokemon.
Friendly battles are available at any time and all of the modes available in campaign mode are available there as long as you've played it once.