I've noticed a lot of cute places along this stretch currently, but many are on the higher-end, price-wise (I'm thinking of both retail options like the wine store currently on Mass Ave, Flour cafe, the VIM fitness center, etc). Would be great to see some more affordable options for "normal" folks along this stretch, as well as for the students nearby (MIT students living on/near campus and students from other schools who commute along Mass Ave to get across the river each day) who spend lots of time traveling along this route.
What do you think about a winter farmers market? The Central Square Farmers Market (Bishop Allen Dr. & Norfolk St.) is outdoors on Mondays May-Nov. The nearest alternatives are the winter farmers market in the Atrium @ 100 Technology Square on Wednesdays and the year-round produce stand (operated by Russo's) in MIT's Stata Center on Tuesdays.
Something I think we need more of and will need more of in the future are spaces for freelancers to work. The current coffee shops and co-working spots in this area are crowded or don't offer wifi. Offering these spaces would boost sales for surrounding businesses too (people want after work drinks and lunch of course.)
Right now, Kendall is an awkward neighborhood to live in: there's no pharmacy or grocery store close by (as other commenters have mentioned) and while it's great that it's close to Red Line access, it feels more transient and business-focused than residential. With so many companies located in Kendall, having more nearby housing (especially units that are affordable to a range of living situations) would be great--as long as the added housing units come with retail that will benefit the neighborhood overall. Adding a grocery store and pharmacy would mean people don't have to leave Kendall to get daily/weekly errands done, and create a better sense of community in the neighborhood.