much worst 1915 for Russia. Central Powers capture Tbilisi and Riga and Minsk

what if Central Powers hit Russia harder in 1915. capturing Minsk and Riga and Tbilisi at end of 1915.

could that cripple Russia in 1916 ?

Capturing Tbilisi in 1915 was pretty much impossible. Even when the Ottomans were advancing, the caucuses campaign was a disaster for them. It effectively destroyed the 3rd army through attrition and the advantage that the defense naturally has in World War I. They'd have likely better off just stopping at a defensible line and holding there. Atleast then they'd not waste huge numbers of troops advancing at an enemy that had a much greater manpower reserve and ability to soak up casualties. Holding the line at a defensible spot also opens the door for some more innovative use of partisan warfare (the ottomans had ties to Azeri rebels and the Germans eventually backed Georgian nationalists in the Russian civil war, both of which aren't bad ideas for pursuing a war in the caucuses without actually having to fight in that logistical nightmare). Pursuing that strategy they might be able to march into Tbilsi 1917 as Russia falls to revolution, and the army splinters and collapses.

I could easily imaginee. Scenario where the ottomans march into Tbilsi and Baku alongside rebel armies that intend to set up an independent Georgia and Azerbijan (with former as a republic much like the actual Georgian Democratic Republic, and the latter with an ottoman supported Shiite monarchy).

And as for the more north eastern part of the eastern front, that's not much further than the eastern front went OTL. So it's likely achievable but it thus also wouldn't be particulalry earth shattering. The German advance in general in 1915 did about as much damage as it could possibly do anyway by overrunning Russian Poland. It was a massive success for the Germans, and anything past that would probably be gravy and might over extend their lines from what their logistics could sustain. Russian Poland was one of the jewels of the empire, it was one of its richest and most industrialized provinces. They probably expected to be pushed back there given the borders not exactly being favorable, but to be totally pushed out is a hard defeat to top in the minds of the Russian aristocracy and military leadership.

NATO1949 said:

what if Central Powers hit Russia harder in 1915. capturing Minsk and Riga and Tbilisi at end of 1915.

could that cripple Russia in 1916 ?

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Beside getting all these 3 achievements in 1915 oscillates between highly unlikly to impossible - given OTL development of military actions 1914 and 1st half 1915 ...
Yes,
since IMO these could have only be done by even heavier losses of russian troops.

But, ... if ...
these cities were captured due to a change in strategy by Russia, returning to the 'Napoleon beating strategy of our hero Kutusov', means an even greater and deeper defensive retreat in 1914/1915 ...
No,
it might put Russia in a much better position to 'come back' in 1916 with the CP forces extremly streched and thinned out.

In general I would say that beyond the Caucasus, the central powers actually did about as well as they possibly could be expected to in this period of the war. Even in OTL the line ended up being very close to Riga, Minsk, and Tbilisi. In fact the ottomans being more realistic in how they approached the caucuses campaign would probably do the most to hurt Russia further, because the turn around in Georgia was a big boost to Russian morale, especially because after awhile it became the only front they were winning on (until after the revolution they were losing there too).

In general, the central powers did about as much as they could be expected to achieve victory in the east as soon as possible. The war was just sustained for longer than could ever be considered reasonable by Russian government. Which absolutely destroyed itself twice over in their insistence to not accept defeat. Had the provisional government not been an incompetent wreck they'd have requested an armistice by March. But they instituted a war policy deeply in denial of the fact the Russian army was incapable and unwilling to keep fighting the war. Which only made the conditions of the ultimate peace that much worse when the Bolsheviks took power and asked for peace.

From the perspective of winning the war for the allies, it was admirable in that it prevented the Germans from moving the hundreds of thousands of troops in Russia to the western front for almost a full year longer than could have ever been expected. Even the Bolsheviks prevented a withdrawal for a long time by refusing to sign the treaty until it looked like the Germans would be capable of marching on Petrograd.