The case with Mijumaru is most unusual compared to the other Water Pokemon cases. Usually Ash's first regional Starter Pokemon evolve at least once. This was the case with Bayleef, Sceptile, and Turtwig. However, Mijumaru would also go into the main Water Pokemon category which was being discussed and those Pokemon never evolve. It seems Mijumaru is stuck in the middle and only the writers can decided it's fate at this time. There's no real trend to this since there are too many clashing figures to account for. Honestly, only time will tell.
That beta art gave me the impression they were Don George's Pokemon.Based on the beta artwork, there was the evolution of Mijumaru and Tsutarja in it, so that would make me think Mijumaru would evolve at least once, making it Ash's first evolved Water Pokemon.
Mijumaru's mid form TOTALLY suites Ash's personality. Now I dunno if it'll go all the way to it's final form...but as people have said, the beta art showed the middle evo, so I'm sure ash's will evolve at some point