Smash Bros Ultimate introduces some interesting things. The content is significantly higher than any previous Smash Bros game, around triple.
I. Smash X Castlevania
1. It adds in the Belmonts: Simon and Richter, with Alucard joining as an assist character.
2. Dracula joins as a boss character in Castlevania.
3. 34 tracks from the Castlevania series have been added.
II. Content
1. All 75 past characters (including Cloud, Solid Snake, and Sonic) are included. Along with new characters including the aforementioned Castlevania characters, as well as Daisy (Gameboy Mario), Chrom (Fire Emblem), Dark Samus (Metroid Prime). A few more still to announce.
2. Over 800 tracks from various games, and an additional 100 Smash songs (900 total).
3. Boss battles including (the aforementioned) Castlevania's Dracula and Rathalos from the Monster Hunter series. Similar to the Brawl bosses in the Subspace Emissary mode (which was like a built-in 2D platformer) which also included Pokey (Earthbound), Master Hand, Ridley (Metroid), the Giant Pirhana (Mario Galaxy), Giga Bowser (Mario), and Tabuu (End boss, Smash original).
4. 103 battle stages.
III. Modes
1. Brawl's tournament mode returns with up to 32 competitors local, online, and CPU.
2. Squad strike - where each player uses multiple characters rather than just one.
3. Stage morphing - essentially you choose two stages and it will morph between them through the battle.
4. The biggest mode has not been revealed yet, they blurred it out from the main menu and dashboard. I suspect it is tying into their upcoming online service. Most are suggesting it's a story mode.
5. Detailed customization, right down to which music will play and how often it will play.
And a lot more that can be found (mostly) in this Smash Bros "Nintendo Direct" video.
I. Smash X Castlevania
1. It adds in the Belmonts: Simon and Richter, with Alucard joining as an assist character.
2. Dracula joins as a boss character in Castlevania.
3. 34 tracks from the Castlevania series have been added.
II. Content
1. All 75 past characters (including Cloud, Solid Snake, and Sonic) are included. Along with new characters including the aforementioned Castlevania characters, as well as Daisy (Gameboy Mario), Chrom (Fire Emblem), Dark Samus (Metroid Prime). A few more still to announce.
2. Over 800 tracks from various games, and an additional 100 Smash songs (900 total).
3. Boss battles including (the aforementioned) Castlevania's Dracula and Rathalos from the Monster Hunter series. Similar to the Brawl bosses in the Subspace Emissary mode (which was like a built-in 2D platformer) which also included Pokey (Earthbound), Master Hand, Ridley (Metroid), the Giant Pirhana (Mario Galaxy), Giga Bowser (Mario), and Tabuu (End boss, Smash original).
4. 103 battle stages.
III. Modes
1. Brawl's tournament mode returns with up to 32 competitors local, online, and CPU.
2. Squad strike - where each player uses multiple characters rather than just one.
3. Stage morphing - essentially you choose two stages and it will morph between them through the battle.
4. The biggest mode has not been revealed yet, they blurred it out from the main menu and dashboard. I suspect it is tying into their upcoming online service. Most are suggesting it's a story mode.
5. Detailed customization, right down to which music will play and how often it will play.
And a lot more that can be found (mostly) in this Smash Bros "Nintendo Direct" video.
“I would unite with anybody to do right and with nobody to do wrong." 
