Why’d monster hunter brainstorm timetravel to the specific era the story takes place?
Will the others ever see his alt mode?
The reason is the same as in canon - he wanted to save Quark.
Okay, I'll try and keep this short and sweet.
Brainstorm lives in the far future and is happy with Quark, until one day it turns out that Quark has a fatal spark disease that will kill him if nothing is done about it. They of course go to the hospital, but it turns out that only certain types of the disease are treatable and modern science still can't do anything about it.
Long story short, no one knows how to cure Quark's spark.
Brainstorm, as a true scientist and a good conjunx, naturally begins to research the subject himself and stumbles upon some strange information. All the sources, studies and records on the study of this disease go back a long fucking way. All that modern scientists have been doing for the last million years is just improving and refining the method of treatment, which was invented in absolute antiquity.
Brainstorm investigates further.
He discovers, all the original research records belonged to a mech named Perseptor, who amongst many other things was studying sparks. And it's when Brainstorm manages to get his hands on copies of these very original records that he finally realises why no one has been able to take this research any further. The records are very well structured, detailed and accurate, but half of the information is taken out of nowhere. The Perseptor specifies the types of sparks that certain substances affect in certain ways, but nowhere does he mention where he got this information from. He might, for example, write that certain types of sparks tend to develop internal micro-cracks when exposed to certain factors for long periods of time. And Brainstorm, having read that, can only stare blankly into space, because yes, micro-cracks in sparks is something that exists. But even in his time, there's no equipment that can detect them if they're INSIDE. So how the hell did an ancient mech with his primitive tools figure all this out???
His curiosity isn't satisfied. The research just cuts off in the middle, as if the mech that did it just abandoned it or died suddenly.
Brainstorm, like many scientists before him, tries to start his own research based on the information pointed out by Perseptor, but finds himself at the same dead end as all the medicine of his time. He just doesn't have the same mysterious way of collecting data that this...Perseptor had.
And Quark isn't getting any better
Eventually, Brainstorm comes up with a brilliant idea. What if, instead of trying to find a cure, he just (ha! Just.) went back in time and saved the dude who was definitely going to invent the cure but didn't have time? He decides it's genius and creates a time machine.
He goes back in time to find Perseptor and well, he gets a surprise. Turns out the dude who researched spark disease was a spark eater. And also on the verge of starvation, but Brainstorm finds a way to help him, it's all good:) It turns out that all this time, Perseptor didn't have any mysterious equipment to analyse the sparks, he was the equipment himself. In fact, he didn't specify the sources of his findings for the research, because the phrase ‘I figured it out because it tasted different’ sounds incredibly compromising and would have signed Percy's death warrant if his notes had fallen into the wrong hands.
Next, I'm not sure how it would have developed. I think as the story progresses, Perseptor and Brainstorm work together to invent a cure for Quark. And then, if you like to cry, Brainstorm goes back to the future and cures him, and Perseptor stays in the past.
If you want adventure, Brainstorm could take Percy back to the future with him. Quark would be really fucking scared and confused at first, but they'd figure it out quickly and conjunx Percy into their futuristic fluffy pairing. (Also, I have a lot of fun thinking about Brainstorm and Quark showing Percy the advances of future science, and the future world in general.
Also, I think Brainstorm would do a good job of hiding his alt mode while he was in the past, but a couple of times would use it to escape from someone. One time he'd also give Percy a ride, and I know Percy would be incredibly freaked out by the breakneck speed that jets can achieve ahahaha
That…wasn’t as short as I wanted…..my inner fic writer took control