As i was saying, i would have been interested in a DC/TF crossover. i don't know as much about DC's stuff as i know about Marvel's, and i don't know as much about Marvel as i know about TFs, but there are plenty of characters and/or storylines that interest me under all those banners.
Countdown linked an article that includes some great sample artwork--the illustrator(s) are very skilled, plus they're obviously drawing some cool characters, and furthermore drawing cool characters interacting with other cool characters whom they would never normally be able to. These are some of the best reasons to even have crossovers! Not to mention the written aspect of "what if"--what if these characters met, what if they lived in the same universe (or multiverse)?
Another thing i like about crossovers is that they're often a great choice for people like me who are obsessed with continuity, practically "allergic" to spoilers, but have fallen behind on keeping up with the main storylines. Crossovers are often non-canon (or what TFWiki calls a microcontinuity
), and even when they're considered part of the main canon, they still have to be written in such a way that's understandable even if you're only familiar with one of the two or more sources being integrated into the crossover. The stories can be pretty exciting--how did these characters wind up in the same universe, let alone working together and/or fighting each other? and the main plot must have some pretty high stakes if the major players from both of these (normally) separate series have (or have to) come together to deal with the situation--but at the same time, the plot doesn't really spoil anything from the main storylines, because the crossover has to be expendable so as not to lose customers (from either fandom) who decide against this crossover in particular or all crossovers in general. (Like, if Superman sacrifices himself to destroy Unicron, TransFans who don't want to read the crossover will all be saying, "Wait, what happened to Unicron? How did the Transformers possibly defeat him?" and all the DC Comics fans who don't want to read the crossover will be like, "How come Superman isn't in these anymore? Why did he just suddenly disappear?")
But perhaps the best thing about crossovers is that they usually do not come into existence without the involvement of some major geeks (i mean that as a compliment), which means the characters are presented by people with personal firsthand knowledge of what fans like about the characters, so the characters stay in character, so to speak. For example, in the sample artwork in the article from Countdown's link, Aquaman isn't just randomly fighting some random Decepticons who just randomly happen to be in the water--he's fighting the Seacons and Submarauder. None of them (Aquaman, the Seacons, or Submarauder) are characters i know well or focus on, but i appreciate that someone put some thought into the fact that, if the TFs and DC Comics characters were to wind up in the same universe, this is the kind of matchup that would happen.
That being said, i'm not so sure about Optimus Prime becoming a Green Lantern. From a storyteller's perspective, of course--who else would they choose? But while the artwork is pretty cool, and the green doesn't really clash with the red and blue, it's just that Prime's deco is so atypical compared with other Green Lantern uniforms' color schemes--especially with Optimus in the Green Lanterns' group shot included with that article.
Still, too bad the idea got shot down.