By the rules of non-downtime Independent Research:
A wizard can also research a spell independently, duplicating an existing spell or creating an entirely new one. The cost to research a new spell, and the time required, are left up to GM discretion, but it should probably take at least 1 week and cost at least 1,000 gp per level of the spell to be researched. This should also require a number of Spellcraft and Knowledge (arcana) checks.
This implies that a Wizard can duplicate the effects of any spell across any spell list, RAW, and add that spell to his spellbook. This follows from both "duplicating an existing spell," as those spells exist on other spell lists, and "creating an entirely new one," as those spells do not exist on the Wizard spell list.
Of note here is that no part of the Magus class entry allows them to add new spells to their spellbook by any means other than leveling up. Here are the lines applicable to a Magus learning new spells:
A magus may know any number of spells.
At each new magus level, he gains two new magus spells of any spell level or levels that he can cast (based on his new magus level) for his spellbook. At any time, a magus can also add spells found in other spellbooks to his own.
A magus can learn spells from a wizard’s spellbook, just as a wizard can from a magus’s spellbook. The spells learned must be on the magus spell list, as normal. An alchemist can learn formulae from a magus’s spellbook, if the spells are also on the alchemist spell list. A magus cannot learn spells from an alchemist.
Another way that the Magus can probably learn spells RAW is via Knowledge Pool. See this question and its accepted answer for details, though this may change from table to table.
RAW, the Magus cannot research spells. However, it seems reasonable that RAI this prepared caster with a spellbook can research spells like a Wizard, and indeed this would appear to be a common house rule.
Under the assumption that this house rule is in place, the Magus then falls under the same category as the Wizard with regards to researching spells.
The logical conclusion from this is that if the Magus can research, the Magus can then research spells from any list, with the most obvious and least-controversial non-Magus list being the Wizard's spell list.
Would it be unbalanced in a typical campaign to allow a Magus to research Wizard spells?