1.Stats: Should Tauros get a stat boost, Evolution, Mega Evolution or nothing?
Tauros is probably the poster child of power creep. Originally one of he strongest Pokémon in the game, it became significantly less useful in later games, and became an earlier mid-game Pokémon, and eventually a late-early game Pokémon in Sun and Moon.
With this in mind, Tauros is theoretically capable of all three options:
A Mega Evolution would require a good Ability and possibly a type change to work, but it can work.
An Evolution would have to be in the league of Electivire and Rhyperior, slower but more offensive.
A stat boost can easily work, as Tauros' stats are fairly mediocre by today's standards, though still decent. It can only have +50 more stats for both an evolution and a stat boost however, and a Mega would probably work best after a stat boost to future proof the bull.
2.Should Tauros get more abilities?
Tauros has a full three Ability set, and they are all pretty decent. But since we are talking about other possibilities, Reckless, Rock Head and Rivalry sound all good.
3. Should Tauros get more moves?
Yes, though largely for its in-game movepool which is pretty dry outside of Payback and Zen Headbutt.
Ignoring that, Poison Jab, Swords Dance from TM, Aqua Tail from Move Tutors, and a wide variety of Normal and Dark moves such as Headbutt, Double-Edge, Body Slam, Beat Up, Assurance, Trump Card, Tri Attack and other moves like Fissure, Magnitude, Head Smash, Megahorn, Revenge, Rock Blast, Drill Peck and Jump Kick can be granted.
Other moves like Flame Charge, Avalanche, Bulk Up and Superpower would be good as well.
4.What additional typing Tauros can have?
Ground and Rock are the most obvious ones, but Steel and Electric can also work. Dark, Fighting and Psychic would be lesser options as well.