OK, I did a bit of cursory research to try and figure out which, if any, Dragon Quest characters are most likely. It's maybe a bit too much work to put into a scenario that has mediocre-at-best chances of succeeding, but my day was free so screw it.
It became pretty clear pretty quickly that there are roughly three major competitors for the Smash slot: Slime, the Hero of DQ VIII, and (spoilers for an almost 30 year old game), Erdrick aka the protagonist of DQ III. You can never rule out that they'd include a character from the upcoming game, but I'm going to set aside that possibility for now.
Slime obviously has the benefit of being the highly-successful mascot, and has also starred in a few games of his own (the three Rocket Slime games for the DS). He was also named the most popular monster in a poll of Japanese fans, but I doubt that was much of a surprise.
The big surprise for me was Erdrick, who I was at best vaguely familiar with before looking up this stuff. Dragon Quest III was massive over in Japan on the Famicom, and has continued to sell well on rerelease, selling the most of any Dragon Quest (including rereleases). Most importantly, in my eyes, he was named Japan's favorite character in the poll mentioned earlier. How this poll tracks with the game-playing public at large is hard to say, but it definitely indicates that he's extremely popular among the kinds of engaged fans who would participate in such a poll. Lastly, he's actually very important lore-wise, serving as a connecting thread between the 3 DQ Famicom games that launched the franchise. If Japanese players are for some reason reluctant to play as Slime I would hesitantly predict Erdrick would be the pick.
The DQ VIII protagonist benefits from a few things, however, that merits giving him another look. Anecdotally, he'd seem to be the most recognizable in the West based on my poor non-Dragon-Quest-Playing self's impressions, he's definitely the character I personally most easily recognize. I doubt that the West is going to drive whatever DQ character is potentially chosen, but you never know if they try to offset the inherently Japan-focused move of a DQ rep by choosing a character not completely obscure in the West. Even if DQ III has sold the most including rereleases, it looks like DQ VIII has sold the most of any single sku of the PS2, and I don't think the source I'm looking at includes the 3DS remake, so it very well may be the most overall by now. The DQ VIII protagonist also scores respectably on the Japanese fan poll I mentioned earlier, 5th of all characters which makes him second among all protagonists.
One thing that puts a damper on all this speculation, however, is that according to SourceGaming's reporting on Japanese response to Smash DLC there really isn't any burning desire to see a DQ character in the game. They mention a lot of caveats in their piece, and there are definitely some things that seem "off" about the list which makes me think Japanese Smash fans might be just as likely to be victim to groupthink on what is 'likely' as those of us overseas, but it's definitely something to pay attention to. No poll, furthermore, can adequately answer the basic question of whether a character seems the "obvious" choice, you really need to ask people on the ground to get a better sense of that. Even if he didn't win every poll (which he didn't!) Cloud was the "obvious" choice for "most iconic Final Fantasy character" even if you didn't like him. Likewise, Erdrick could be just barely more popular among a host of other choices, or another character not even mentioned could be 'iconic' in a way that's difficult to poll.
Sources:
Japanese Dragon Quest Character Survey
Dragon Quest Series Sales
SourceGaming on Japanese Smash Requests
Final Fantasy Male Character Japanese Survey
That was probably too many words for summing up the most basic of research, but I just wanted to get my thoughts down. If I'm off-base or being stupid about any of this just let me know. As I said I knew next to nothing about Dragon Quest (the only significant time I've sunk into a DQ game was Dragon Quest: Monsters on the GBC), so I'll defer to those who know more than me.