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This is what I've thought it the most likely scenario going forward.  It's better than one alternative.

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Negotiating leverage, and I'm not sure this really is a "massive buildup", it is a buildup, but as you note he's had tens of thousands of troops stationed near Ukraine for years.  Perhaps he wanted to get them noticed by moving 20,000 or so to a forward position.  They have to be somewhere, so the expense is in building quarters for them nearer Ukraine.  So, it's a gambit, and also provides him with a credible threat to invade, which probably is in the back of his mind.

He could well claim ethnic Russians in Donbass are being persecuted and move into that region.

The question is, why spend all the money for this recent large military build-up along the Ukrainian border at all?  Clearly he's pushing for something different, something new.  What's his goal?

He's already claimed this.  If he "moves in" any more than he already has by providing Russian backing to separatist rebels, it's an overt act of war and an invasion of a sovereign nation.  And it will be treated as such by the other sovereign nations nearby.

The Ukraine-Polish-UK announcement earlier this week is essentially a line drawn in the sand.  Ukraine doesn't have NATO affiliation, but the UK has officially positioned them as a Western ally and although they may not be what they once were, but the RAF is more than capable of establishing a no-fly zone over much of Ukraine and the RN is capable of neutralizing Russian naval forces.