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I'm thinking of making a homebrew magic glaive with a 15ft reach rather than 10ft.
What rarity should this be?
Also, would making this attunement or non attunement change its rarity?
For example, you need to attune to it to learn how to handle the extra weight of the glaive.
Rarity assignment is a little inexact, given the range of capability or power that one finds in items of the same rarity. Broom of Flying as compared to Winged Boots is but one examples of the range for uncommon items, which leaves one puzzled at why a potion of flying is rare and is a 'use once' consumable to provide the same kind of function the others provide consistently.
A standard weapon +1 is uncommon rarity, but as you have not added a + to the weapon, it could be "common" if we compare it to the Moon Touched Blade from Xanathar's Guide to Everything. That item has one magical feature - to cast moonlight when unsheathed - and does not require attunement.
Extending the reach, however, could be tactically significant in terms of how much more of the battlefield your wielder could reach out and touch. Uncommon is close enough to account for that, since it expands the range of both melee attacks and opportunity attacks.
Probably not a requirement, however, your rationale for the attunement - to really internalize how to handle this otherwise unwieldy weapon and benefit from the magic - is a good enough reason to call for attunement. That requirement prevents enemies or allies from 'just picking it up and using it' if the weapon is dropped, the wielder is disarmed, or the wielder goes down to 0 HP. While attunement represents a slightly higher opportunity cost for wielding this weapon, there is a small benefit to it in preventing it's full use at the drop of a hat, or in this case, at the drop of a weapon.
Uncommon rarity, require attunement. It's close enough, given the range of capability that category of magic item holds.
If you review the magic item rarity tables, major and minor, in Xanathar's Guide to Everything, your guess is as good as mine regarding this proposed weapon being a minor or major Uncommon item. Is it as good as or better than Gauntlets of Ogre Strength? It would take a lot of play testing to get a feel for that, but my initial estimate is that the nod goes to the gauntlets for lower level PCs. In the hands of a Cavalier (Fighter), though, or a player with the Sentinel feat, this weapon might be very powerful in some tactical situations in terms of who it can reach for an opportunity attack and / or attempts at 'lock down'.
