If it was mainly for fostering better relations, there are plenty of things that would be far more useful to China rather than Su35s. Systems like the S400, Mi28 or Ka52 attack helos, especially the navsal variants, would all be far more useful for China than the Su35 ever will be.
But increasingly, the balance of power is reversing, and there are more and more things that China makes which would be very useful to the Russians. Already, Russian firms are sourcing more and more important components and subsystems from China because they are both better and cheaper than what the Russians can make themselves, and it is mainly Russian pride that is preventing the purchase of entire systems and platforms.
An area where China clearly ahead of Russia is in the field of UAV and UCAV. The Russians had to buy Israeli UAV not long ago, and once the massive array of experimental UAVs China is working on right now start entering service, there will be plenty that would be useful for the Russians, especially in Chechnya.
Chinese shipbuilding is also head and shoulders beyond what the Russians can manage in pretty much every measurable field. It would make a lot of sense for the two to collaborate on things like LPD, LHD and carrier designs to come up with a common hull that China can build on time and on budget, into which both can install their own sensors and weapons. If Russia wants to field a new carrier before the next decade, their best bet would be to get the Chinese to build one for them.
Similar thing with carrier aircraft. The J15 is at least a generation ahead of the Su33s the Russians currently field, and their size offers inherent advantages over the Mig29/35s the Russian navy are planning to get. And tbh, I think one of, if not the biggest reasons for the Russian navy choosing the Mig is the fact that it is in production and the Indians had already paid for the modernisation. Had China gone with the Russian Su33 offer, I am pretty confident the Russian navy would have piggybacked on that deal and bought new build Su33s instead.
The J31 looks like a perfect candidate for developing into a carrier borne fifth gen, and China is also working on carrier based AWACS/AEW and almost certainly carrier borne tankers in the near future. All of these planes would be of much use to the Russians.
If you cast your eyes to the future, there is indeed much scope for Sino-Russian military co-operation, but almost all of that involves the Russians buying into Chinese projects, not the other way around, and that is the biggest obstacle to closer military collaboration between the two. There is currently no external threat grave enough to get the Russians to swallow their pride and buy Chinese weapons, and the Russians simply lack anything good enough to tempt the Chinese to want to buy.
Cost is a factor, and yes pride too, as I am sure the Chinese have had more than enough of the brainless charge that all they can do is copy, but so are performance factors. Why would the PLA want to spend more money buying a worse platform? Even if the performance of Russian platforms are on par or even slight better than what China can make themselves, the different munitions, operating frequencies and often, the languages of the Russian systems present problems and shortcomings that will usually more than outweighs what little performance advantage they might have.
Logistics wins wars as surely as soldiers and weapons, that old adage is as true today as it has ever been. Adding a token sized force of Russian system would just complicate logistics for very little actual capability gain.
On the modern nerworked battlefield, you either win as a unit or die as an individual. How well does anyone expect Su35s to be able to network with all the other Chinese assets that would be operating in a war zone? I seriously doubt that the Su35s would even be able to datalink with other Chinese assets, never mind more high level things like co-operative engagement or taking advantage of friendly EW assets etc.
For those Su35s to be of much use, the Chinese would either need the source codes to fully integrate them with other Chinese networks and weapons, or strip out much of the radar and avionics and replace them with Chinese kit. Either option will take time and money to implement, which again makes it seems far fetched that China would bother for so few aircraft. Especially since the Russians would almost certainly never share the Su35's source codes, but if you strip out the radar and avionics and replace them with Chinese stuff, you end up with something a hell of a lot like a J11B with overcharged engines. Far better to just put better engines in J11Bs.
The days of China buying Russian arms are over, if they two are to develop deeper military ties, both must accept this new reality and make the most of it. But that almost certainly means Russia would become the junior partner and end up buying Chinese designed and manufactured systems enhanced with Russian components and subsystems, but that is just something they are not ready to stomach right now.
