First of all, DoS systems are out of the table. The game will use official 5e rules and lore for everything that those rules cover, like summoning(They have WoTC collaborators working with them to make sure of it), with some adaptations to a videogame, or at least that´s what they said in all the interviews and gameplays.
Second, BG3 it´s a videogame. I doubt you will have the +100 druid/polymorph shapeshifting forms nor the +100 creatures you can summon with "Conjure animals" and "Summon fey/celestial/fiend, etc", summon spirit spells or create undead spells; but I hope they will allow the players to have some variety.
Right now the summonning spells and polymorph spells were not showed in the gameplay, so we do not know much about how it´s going to be:
Can you summon outside combat? can you polymorph your summons? can you talk to them if they have a language? Do you need actual corpses to create undead like in pnp? Are they going to nerf the duration of some spells like Awaken(It lasts for 30 days)? etc.
Ed: We were discussing some ideas in this thread the other day too that could apply to your thread too
Summoning builds
