Here's my more serious Shivan Theory...
The shivans are locked in perpetual warfare with an as of yet unseen enemy. This was has raged for countless millenia, possibly even millions of years. Neither side is able to get an advantage over the other as they are both equally matched. So shivan stratagist hit upon an extremely unorthodox idea - a race capable of defeating the shivans would also be capable of defeating their Enemy. Though this plan would amost certainly result in mutual destruction of both the Shivans and their enemy, the Shivan High Command didn't care as long as the Enemy was destroyed. So they set about grooming the younger races until one emerges that is capable of taking down the Shivans and thus also the Enemy.
Stage 1
THese Candidate Races (who lived well behind the Shivan Lines) must meet serval criteria:
1) The Race must be spacefaring (obviously)
2) The Race must be subspace capable
3) THe race must be agressive, warlike and capable of sustaining a long term intersteller war (mimimum duration 3 Shivan stellar cycle (roughly 15 years))
If Stage 1 criteria are met, then the race is moved to:
Stage 2
The candidate race is subjected to an intelligence test.
A Lucifer Planet Killer with task force attacks the candidate race and begins systematic destruction of the race. However, the Lucifer has a design flaw. If the race is intelligent and/or resourseful enough they will find the flaw and be able to use this flaw to repell the attack.
The Lucifer's flaw is 2-fold
1) It is powered by 5 reletivly vunerable Reactors
2) It's shields do not function in Subspace (though this is a natural flaw as Subspace and Shield Tech are incompatible
Stage 3
After stage 2 is complete, the candidate race is then given time to rebuild and conduct R&D. after a certain amount of time, the Shivans attack the candidate race with a more advanced fleet to see what has been learned.
Stage 4
If the Candidate race is advanced enough to be able to repel this more advanced fleet, then a Sathanas is dispatched to engage the Race.
Stage 5
If the Candidate race is capable of destroying a Sathanas, then a Sathanas Sun-Buster fleet is dispatched to destroy one of the major stars of the Candidate Race's domain. THe purpose of this is a Reaction test to see how the Candidate Race deals with the incursion.
Stage 6 and beyond.
Details unknown. But it continues untill one of the Candidate races advances enough to become the Shivan's final solution
THe Ancients met the stage 1 criteria and were moved to stage two. The Ancients discovered the solution but were exterminated before they could take advantage of it
in the Earth year 2335, the Terrans and Vasudans were moved to stage 2. AT first they were just another two candidate races albeit slightly more challenging ones. What caught the Shivan's attention was that the Terrans and Vasudans previously, blood enemies, quickly ceased hostilities and allied themselves against the invasion - this was something that had never happened before. Together they quickly reverse engineered shivan sensors and shields and developed new weapons capable of penetrating the shields of Shivan fighters. Though the Shivan's rendered Vasuda Prime uninhabitable, the Allies continued to develop.
First they developed miniturized intersystem jump-drives for fighters, then they discovered the design flaw in the Lucifers reactors. However, the Allies were unable to take advantage of it untill they discovered the Remains of an Ancient City. Here they found out how to track an object into subspace and take advantage in both the Lucifers flaws to destroy it -
As the Lucifer was destroyed at the very last minute, the explosion destroyed the Jump nodes in Sol, cutting of Earth from the rest of the galaxy. Shivan high command decided that Sol was out of the Game and would be ignored unless it reconnected to its colonies.
Despite the loss of both Homeworlds in one way or another, the Allies were able to repel the shivan attack and thus were moved to Stage 3.
However, when the NTF discovered the knossos portal and encountered the Shivans, they triggered the Attack phase of STage 3 early. Left with little choise, the Ravana task force began its work. THey discovered that Allies had developed beam Weaponry amongst other things.
Because the Ravana Task force was discovered early, it was forced to initiate the attack phase of stage 3 before it reached full size. Thus as the Ravana Task force was not at full strength, it was decimated quite quickly and as one can expect the GTVA were moved to stage 4 way ahead of schedule.
The GTVA barely managed to stop the Sathanas managed to pass stage 5 with a Pyrrhic victory - sealing the Sun-Buster Fleet in Capella at the cost of many of their best ships
While this was going on, the shivans were very supprised to find one Terran ship contact them. Bosch and his Senior Staff were taken away for reasons and purposes unknown (I suspect modification like B5's vorlons did with Lyta or like what the Shadows did for Technomages) and the rest of the crew were killed. The reason being was that direct contact prior to Final Stage was forbiddon.
Following the Capella Debacle, Shivan High Command ordered a withdrawal from GTVA Space for an unspecified amount of time. The GTVA would use this time to develop Knossos and ETAK tech and further develop it Weapons Tech and understanding of Shivan Tech.
The Ultimate outcome of the Shivan's plan is unknown. The GTVA showed promise, buy may just be one of many species at that level. Ultimatly, The GTVA may not even end up being the Shivan's Final Solution but only a stepping stone.
My basis for this theory
1)

's comments that the Shivans were symptoms of a bigger problem
2) Neither Terrans, Vasudans or the Ancients encountered the Shivans till after they had discovered
3) Everytime the Terrans and Vasudan's have encountered the Shivans, their tech has advanced more. -