Premise

Tithing Plains is something white should be able to do—cards like Gift of Estates, Tithe, and Oreskos Explorer provide an unique gameplay element without affecting the color pie adversely as they don't really provide color fixing nor "true" card advantage for . By "true" CA I mean that this way can't draw it's threats and answers. Also, in all likelihood it has the least amount of unique mechanics, so any potential distinct things for it to do should be valued very highly.

However, I also think that could use that fetching element without the tithe restriction. This also reduces complexity in general here making this an aspect of the pie that's more readily usable at lower rarities.

I would much rather have a colorshifted Gaea's Bounty in rather than cards like Bygone Bishop, Mentor of the Meek, Kor Spiritdancer, Mesa Enchantress, Puresteel Paladin, and Sram, Senior Edificer.

Notes

This would not allow to ramp or to fix its mana base.

Color Shifts

• Gaea's Bounty

Precedent

• Safewright Quest
• Endless Horizons

Examples

• Open Plains
• Heaven's Arc
• Twilight Harvest

You could add Safewright Quest as a quasi-précédent.

Personally I'm a fan of all of your card draw examples. In the midst straightforward situations, with only one on the battlefield, they're just card-for-card. Mentor of the Meek is the one that breaks this most easily, with tokens. But white needs at least a little help in that area, or it just drowns. It's already a bit difficult to play mono-white in EDH without at least that much.

Added Safewright to the list.

Note: Pretty much all those cards listed as "precedents" are more like "quasi-précédents".

When it comes to card draw in I'm definitely not a fan of that: in a sense this is suggested as a replacement for those cards - as an "alternative way". However, all of those draw engines are currently rare since the idea is something that's being tested by WotC. If they were to remain at rare, then it's plausible that this concept could co-exist with them.

IMO "running out of fuel" is like the sole weakness the color currently has since both it's threats are powerful and it's answer are extremely flexible. CA is the only department where it lacks - and this is something that should remain in one manner or other. You know, each color needs a weakness to characterize it.

land search can't be distinctively white since green is the main color in this area.

white is the color of "equality" and "equity". this excuse should allow it to catch up to other players who are more wealthy in any given area. Like white gets to land grab if that allows it to catch up to other players who have more lands. then expand this concept to hand size, life totals, creatures, etc.

there's your unique white mechanic. works better if white player is behind, so not too advantageous for white.

ex. If you have fewer lands than any player, tutor a Plains. Else scry 1.

Gain 3 life. Then if you have less life than any player, gain 2 more life.

I agree with @amuseum. Much like how Black can do things that it normally can't do if it pays life, I feel like White can do things it normally can't do if an opponent has more of it than them - minus card draw. I'd prefer the Plains fetching be used only if you have less lands than the opponent, or if you couldn't fetch in multiples. I don't think cards like a colorshifted Gaea's Bounty would add much to its identity or its gameplay - unlike Green, it doesn't care particularly much about Plains, Armored Ascension aside, and control decks would just rather run removal in place of it, IMO. Since white rarely has large creatures and its weaknesses in mana consistency and card draw make up for its universal removal capabilities and strong early game creatures.

­ doesn't need to distinctive in this and it was never aimed to be. It would be just another niche within land fetching ability as far as color pie is concerned.

While equality effects certainly fit and are , they are pretty hard to pull of at common in a satisfying manner.

Ie. "If target opponent controls / has more X than you,"

takes already a line so you are left with two lines of text to not invoke red flagging. Also, those kind of cards can end up being entirely dead in that creating the scenario where you could gain advantage of them isn't worth it.

I fear that since these kind of effects are already within 's pie, it would seem reasonable to me that we would have seen more of them if they were satisfactory.