With a larger player base I'd be in favour of such a matchmaking idea. My concern though is that you'll only end up with a small number of people queuing at a time (except for very busy periods) similar to the existing matchmakers, and you'll inevitably have some people who are very good at whatever custom mode is highlighted compared to a new player.
That said, one of the biggest issues I have with both the 1v1 and 2v2 matchmakers is that the maps are too much like hard work. That is, if I get a 1v1 map there's a good chance it's some large sprawling map with mexes everywhere that requires me to do a crazy amount of actions a minute to keep up with t1 land spam being mandatory (except on huge maps or naval ones), which ends up being stressful not fun. In 2v2 from what little I've played it's similar, but with the added pressure of a teammate who will be angry that I don't perform as well as they'd like.
I think this is a large part of the reason games like gap, astro, dual gap, pass etc. are popular - you dont have to focus on as many areas of the map at once, and so there's more chance to just have fun, and it's alot easier to get into as a new player. Even setons and isis mirror this to a large extent (ignoring the players who will make your life miserable if you don't conform to the expected way of doing things when starting in a particular slot).
Therefore I like the idea of having lots of different 'fun'/different types of modes that will make a change in gamestyle from the standard 1v1, especially if they include vs AI challenges, I'm just not sure how to solve the issue of too few new players wanting to do it leading to those that do try getting crushed by pros. One possibility might be to showcase it on the news page (e.g. you have a different mode each week, and the news page features it prominently).