1.Stats: Should Stantler get a stat boost, Evolution, Mega Evolution or nothing?
Stantler is an enigma to be sure. It has only been available in-gae for its original game, GSC and its remake, as an early mid-game species. The rest of the games it appeared in it only existed as a post-game Pokémon, and even then it only appeared once a generation. It did have a brief stint as a version counterpart to Kecleon in Diamond and Pearl, which is nice. It is also slightly stronger than Girafarig which is also nice.
The highest stat boost Stantler can get is +70, as it has the same BST as Magneton, 465. This means it can theoretically get all three options, with a stat boost being at least +20, an evolution being at least +70, and a Mega being +100.
But it should get something, because Stantler is so terrible, it doesn't even have a stat beyond its 95 Attack.
2.Should Stantler get more abilities?
Stantler already has three abilities, though only two of them are good, with Frisk being mediocre.
Other possible abilities would be Anger Point, Illuminate, No Guard, Reckless and Trace.
3. Should Stantler get more moves?
The most obvious move for Stantler would be Swords Dance. Outside of that, it can get Elemental Fangs at it has Bite, Horn Drill, Horn Leech for its horns, Screech, Supersonic, and Hyper Voice as it gets Uproar, Assurance, Double Hit, Fury Attack as earlier moves, Slack Off for recovery, Aqua Tail and Strength for coverage, and Echoed Voice, Dazzling Gleam and Flash Cannon for its 85 Sp. Attack.
4.What additional typing Stantler can have?
Psychic is the most obvious one, followed by Ghost and Fighting, and eventually by Electric and Ground.