Time attack mode also lacks big rings, which give 50 rings if you have all emeralds. So requiring time attack mode for RAs would require nuking the entire scoreboard and starting over. In favour, it is a little awkward that you can't compete in RAs until you have all emeralds; but I think anyone with enough skill to be competing won't find it that onerous a requirement. Against, if we can't get those 50 rings then our stats don't represent the actual best that can be achieved, making the charts feel a little pointless. So I'm strongly against this idea.
In fact, maybe time attack mode should be banned for RA/SA charts? In this mode, some monitors are replaced by ring monitors, so there are more rings available that aren't there in the "real" stages (although this is outweighed by the big rings in any case).
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I think (but haven't really tested) that the lottery in Studiopolis can be re-entered with normal characters; it's just that Super Sonic does it faster. (Incidentally, the 999 stat requires entering a Blue Spheres stage to come out of Super.) I'd really much rather we prohibit using the lottery; that replaces what could be an interesting routing and collecting challenge with rolling against the RNG a couple of dozen times. (And, as you said, there is precedent for this. And we could apply the same rule to Sonic 2 to make those charts work again.)
Other than the lottery, I don't think there's any reason to use super forms in RA/SA. For times, do the same as Sonic 2 and 3: forbid them in the main charts and have a Freestyle chart.
If super forms are disallowed, I don't think entering a Blue Spheres stage makes a meaningful difference, so maybe just allow it?
Definitely limit the signpost points to 100, as in Sonic 3. Otherwise there is unlimited potential score, but getting any score out of it would take hours.
I think there are two more really big questions to consider:
* Should time stats be done in time attack mode? This is obviously more convenient, but it lacks the bosses. I think there's merit in both types of stat, so there should be separate charts for "Time" and "Time Attack" (and separating them wouldn't be an issue, as all the current time stats use time attack, so just move the whole lot over).
* For Sonic's charts, is it okay to manipulate AI-Tails, but forbidden to control him with the 2P controls? This was the case in S3&K. Mania allows a lot more control over AI-Tails (you can do the pick-up-and-fly with just 1P controls), so whether this is allowed or not will make a big difference to charts. I'm in favour of allowing it, because it leads to some interesting routing optimisations. Alternatively, we could have separate charts for S&T and Sonic alone.