Right, I almost always do... But it's difficult to get UMD to work often enough until 10 or so... And that's with traits, feats, items, and point buy... And that's just for wands, let alone scrolls, which can have some quite high DC's.
An archetype with bonuses, the ability to take 10, etc... Would be very nice.
And a class that's what a magus is to eldritch knight but for arcane trickster... That isn't a bard, is INT based... They could never publish anything again and I would be happy.
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How about something like this?
Device Expert: At level 1, a tinkerer adds 1/2 her level to all Use Magic Device skill checks (minimum +1). This ability replaces trapfinding.
Categorizer (Ex): Tinkerers meticulously keep track of their magical and mundane items, always knowing where every item on their person is located at any given time. At 2nd level, a tinkerer never provokes attacks of opportunity when retrieving stored items and any tinkerer who takes the Quick Draw feat may use the benefits of Quick Draw with alchemical items, potions, scrolls, and wands. This ability replaces the 2nd-level rogue talent.
Trapfinding: At level 4, a tinkerer adds 1/2 her level to perception skill checks made to locate traps and to Disable Device skill checks (minimum +1). A tinkerer can use Disable Device to disarm magic traps. This ability replaces Uncanny Dodge.
Device Master (Ex): At level 8, a tinkerer may take 10 on Use Magic Device skill checks even if stress and distractions would normally prevent her from doing so. This circumvents the usual rule that says you can’t take 10 on UMD checks. This ability replaces Improved Uncanny Dodge.
I'm not sure what you're willing to give up, like how much of the Rogue you would want to keep. Personally, I think that this kind of Rogue should still be able to disable traps, so I kept it in there, though a little later in progression. Could probably think of other little things to add in, but then I'd probably be taking away things like Sneak Attack dice. Like something that might be cool is that at 3rd level and every 4 levels there after the Tinkerer loses the Sneak Attack die at that level to gain the ability to be treated as if they know a spell (or a few spells) for the purposes for magic devices from the Sorcerer spell list. At 3rd level they'd be able to choose level 1 spells or lower, at 7th level 2 spells, and so on. Just a thought.
thats about it: a Rogue that is specialized in using Use Magic Device with wands and scrolls.
Sorcerer with Sage Wildbloodline and Seeker archetype. Take additional traits to nab some roguish class skills and voila, rogue with crazy good UMD because you can use all sorcerer/wizard stuff without making a check.
Take a long, hard look at the Pathfinder Chronicler.
8 Skill points per level.
Level bonus to UMD to read scrolls.
The ability to pull any scroll out of a bag, costing up to 100g per level.
At tenth level (15th character) you could have a rogue who tricked into read magic and can cast any 1st-4th level spell in the game given a round to prepare. That's a heck of a lot of utility spells at a moment's notice.
He would have a +10 to UMD for that and any other scroll.
Sure, you lose sneak attack, but I expect any UMD arch for rogue would take a hit there.
It's probably not as good as what you're looking for, but hey, UMD is pretty effing nice.