Haste is a great spell. I want to make it better at the cost of higher spell slots. Additions to the spell description will be added in italics (except 3rd-level transmutation since the book puts that part in italics).

HASTE
3rd-level transmutation

Casting time: 1 Action
Range: 30 ft
Components: V, S, M (a twizzler)
Duration: Concentration, up to 1 minute

Choose a willing creature that you can see within range. Until the spell ends, the target’s speed is doubled, it gains a +2 bonus to AC, it has advantage on Dexterity saving throws, and it gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

When the spell ends, the target can’t move or take actions until after its next turn, as a wave of lethargy sweeps over it.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, you can choose one of these additional effects for each slot level above 3rd (each option can be chosen only once):

• The target’s speed is tripled, instead of doubled, for the duration.
• The target gains a +3 bonus to AC, instead of +2, for the duration.
• If the target has a feature, such as Extra Attack, that allows it to make more than one weapon attack with a single Attack action, it may make up to two weapon attacks, instead of the usual one weapon attack, with the additional action granted by this spell.

For each additional effect chosen, the target cannot move or take actions for an additional turn after the spell ends.

Obviously this makes haste better. But I’m not sure how much better. The +3 to AC could start to push bounded accuracy off the table when combo’d with spells like shield of faith or a build that already optimizes for AC. Basically, there are obvious combos where upcasting in this way can amplify the already brokenness of those combos, but I don’t think it should be much of an issue for most normal use. But just in case, I added the clause at the end to increase the risk of upcasting this. For each additional effect chosen, the wave of lethargy effect increases by a turn. So casting this at 6th level and choosing all three effects means the target is out for four turns when the spell ends.

Some other options to consider for balancing would be to require two extra levels of spell slot for each effect, so getting an extra effect at 5th, 7th, and 9th levels, or having the extra effects kick in when cast at 5th level or higher and requiring 1 extra level per effect, so getting an extra effect at 5th, 6th, and 7th levels. Reviewers that deem this unbalanced as written should consider these changes to slot level for balancing.

Is this addition balanced?