In all honesty, I couldn't see anyone from the 1st or 2nd Gen being able to keep up with the current level of dancing that the group does. From 3rd-8th Gens, none of them were as proficient dancers pre-MM as, say, Yasshii or Ayuyu were, however if they (the previous Gens) had started early enough with the training/practice drills (which I feel is the key to why the current lineup can do what they do), with a lot of hard work they could probably do it. Obviously some members would have a notably harder/more difficult time with it, but yeah, with enough work they probably could have gotten to something like...75-80% of what the current members can do.
In terms of singing, if we're talking about doing it in combination with the current level of dancing that the group does, then IMO, 1st and 2nd Gen would be having even MORE trouble keeping up with it. A lot of the early songs that MM did were not written with much, if any level of "serious" dancing involved. At the most, there was some slow side-stepping and/or some arm movements done while the girls usually stood in place. MM was initially conceived of as a "vocal group" after all, not necessarily a "singing AND dancing" group. At any rate, as is the case with many pop groups, the stronger singers in the group didn't always get the same promotion/attention as the girls who became front girls. And subsequently, the group's front girls weren't always their better singers.
Personality-wise, there's going to be a lot of discussion/debate about how certain members (especially 5th to 8th Gen) were either "dull as stumps" or not given a real chance to shine because they were always in the shadow of/because the focus was always on/because they were never given the same level of promotion as the "Golden Age" (i.e. 1st-4th Gen) members. Others have said/will say that if the younger Gens had stronger individual personalities, they would have/could have been able to come forward on their own to "make themselves known"; and that the fact that they never did is evidence that those members really had little to no "personality". It's an old/ongoing discussion amongst the fandom.
Really, for me, all things being equal, the only former members that I could see being able to "hold their own" (in an overall sense) with the current lineup are Aichan (and I'm not saying that just because she was still around to mentor the 9th Gen and to see the 10th Gen join) and Maki. Singing and dancing was never really a problem for either of them. Between the two of them, Maki was probably the better choice for TV interview/talk segments. For Aichan, sure she was often quiet/uninteresting in TV appearances, moreso in her early years with the group, she slowly started to speak up and come out of her shell a bit more when she took over as leader of the group. And no one can deny that what she lacked in the "interview" department, she made up for many times over in her performance/stage presence she commanded, which, IMO, could surpass what Maki had. As far as the rest of the past members go.....if they were to join MM as it is now, they might have been able to do well in one or two out of the three categories (dancing/singing/personality), but it would be really difficult for them to do even that. Three out of three? I couldn't see many of them even coming close to being able to pull it off.
Risa could/would probably do well, but seeing as how she was still a part of the group when the bulk of the last 3 Gens joined, I didn't think she'd count for this. =P