So here are my reasons for wanting Unova as 5th starter alongside PS2.

1) It's noticeably different to PS2. Just look at the two stages side-by side. The platforms are considerably larger and reach almost all the way to the ledge. It has walls. It's also noticeably smaller than PS2, both in terms of the main ground and the blastzones (except the vertical one is higher).

2) The PS2/Unova layout is one of the most liked and least polarising layouts out there, so having more than one doesn't bring up the same problems as, say, having multiple tri-plats.

3) There aren't suitable alternatives for 5th starter. Lylat has jank. Kalos plays closely to FD with a different edge game.

4) It'll promote variety. There's a good chance that the vast majority (say 40%) of game 1 matches will be played on PS2. Throwing in Unova splits that between the two stages, which already solves the potential problem of visual fatigue which plagued Smash 4. It might also lead to increased playtime on other stages, in those scenarios where the two players can't agree on one or the other and have no choice but to strike.

5) If Unova isn't included in the starters, it's likely banned. I just don't see it working as a CP, it's too neutral and non-polarising (and flat and symmetrical etc). So not having Unova in starters automatically means one less stage in the list.