Ultima being a Dungeon Boss is impossible, its defeated and nuked as endboss from ARR mSQ, unless SE retcons and revives it magically just back into the game...
Sapphire Weapon makes much more sense as a Dungeon Boss, cause we have never seen the skills of Sapphire Weapon, except a kind of Mouth Beam Move, so with Sapphire Weapon would have SE full creativity freedom to make it fit as a Dungeon Boss.
About your FF7 related answer to me, please explain, because I believe I understood you due to language barrier not correcly, so please correct you if I ask you now something wrong, ok.
Did you really just say, and thats how it came over to me as a message, that SE tries in your opinion to screw up the FF7R by changing things, like adding new content and that as a result of that is in your opnion that we will have summons already in Midgar?
If that is really what you said and believe, I have to ask you a few questions:
1. You do know that summons will work completely different in FF7R due to the combat system not being anymore a turn based game, being limited to only 1 summon maximum on field, which is most likely the reason they decided to give us summons earlier?
2.Would you rather have wanted just a boring FF7 that just looks pretty but is in fact just a 1:1 copy storywise of the original, instead of one that uses the advantage of the moment to improve alot of things that weren't just possible over 20 years ago? So would you rather have wanted a simple HD Remaster kind of what SE did with FF8? Are you eventually angry about it that FF7 gets the fullscalle Remake and not FF8?
3. Have you ever considered the thought that having summons already in Midgar over the course of the remake can mean also simply too, that the game will have just at the end due to game design choices this time alot more summons to offer, than the OG had? When SE even gives us from begin on already as pre oder bonus up to 3 different summons, which are all FF8 BASED, don't you believe then that it is quite obvious, that FF7R will most like have more summons later, because the story and content of the OG gets stretched out a bit, due to the game getting delivered over multiple parts, which will be all in itself standalone games the size of normal games with SE still not having an official answer for us, how many parts it will become?
I see summons in Midgar as a good thing, it makes battles against real tough enemies look much more epic and dramatic, plus as said, I believe we will get to see sure a few new summons eventually as replacements in the spots where we originally receive Choco/Mog, Ifrit n Shiva. If I could wish which 4, then I'd go for Fenrir/Cerberus, Diabolos, Pandemona (as Typhoon Replacement) and Siren which would be honestly spoken great additions.