1.Stats: Should Xatu get a stat boost, Evolution, Mega Evolution or nothing?
Xatu is an interesting Pokémon. Initially appearing as a mid-game Flying-type for Johto, it returned again in Hoenn as a Safari Zone species, this time with its evolution being catchable as well. However, unlike fellow Psychic Girafarig, it dd not return for Sinnoh, and since Hoenn it did not came back. While Xatu has some baggage as Will's signature species and as a Magic Bounce owner since Gen V, it is not particularly powerful, due to being inspired by Dodrio who it shares its BST with since Gen VII, and due to evolving early like Johto species, as Johto region was a purposely gimped region.
With its 470 BST it can theoretically fit for all three groups and can get +70 BST at maximum for a conventional stat boost or evolution. However, its pre-evolution Natu has 320 BST, which means the highest gap it can have is 190 BST, which corresponds to 510(+40 BST). With its early accessible nature one can shrink this further, as the highest BST species that evolves at Level 25 and is a two stage species is Phanpy-line, which goes from 330 to 500, which would correspond to around 490 BST for Xatu(+20 BST).
Nonetheless, Xatu will appreciate anything it will get, as it lacks any stat that is +100.
2.Should Xatu get more abilities?
Xatu's main abilities are pretty worthless, but its HA is great. Thematically, Forewarn, Gale Wings, Illuminate, Magician, and Wonder Skin can work, so can Magic Guard, Trace, Insomnia and Keen Eye.
3. Should Xatu get more moves?
Though Xatu has a large movepool, it is largely lacking in power, due to Xatu's status orientation. Moves like Hyper Voice, Uproar, Energy Ball, Flash Cannon, Safeguard, Mind Reader, Meditate, Mirror Coat, Nasty Plot, Reflect Type for expanding its moveset and Acrobatics and Drill Run for sake of flavour can work. Dark Pulse would also be nice.
Xatu can also get some older moves like Air Cutter and Swift to buff its Level-Up moveset.
4.What additional typing Xatu can have?
Normal and Ghost are the most obvious ones.Grass and Dark would be lesser options.