I would have to lean towards not changing type(s) between base and Mega forms. The benefit of having different typings between forms seems to be that you can make use of the base form's defensive capabilities, which could be different from the Mega forms. This walks a fine line with regards to not trying to negate the downsides of a typing; the base form also has lower stats and cannot have too many resistances (or it wouldn't be an undervalued type). The way I see it, if the forms are too different in utility, it risks derailing the concept; if the forms different typings are too similar in utility, the benefits of a type change are minimal. While it would be an interesting exercise, there isn't any merit if it distracts from the concept that the community voted for (as we saw in the concept assessment, this is already a surprisingly difficult concept to get a handle on).
I dislike people's reasoning that if the base form isn't great, that we might as well make a CAP with Mega stats. We should remember that we are looking for a mediocre-at-best typing that has a specific utility in countering specific Pokemon. That already alters how people play the CAP, as looking for the free turn to Mega Evolve is definitely a matter of timing and opportunity cost; the effect against item altering moves, like Knock Off, and generally high BST are a bonus. Yeah, you'll have to use your brain for this concept, even if there are no mind games to play. Finally, the concept isn't "How to best make use of the turn you Mega Evolve", "What are the benefits of a base form being different from a Mega form", "Mega Mind Games" or similar: it's looking at an undervalued typing an playing its strengths and weaknesses by way of being a utility counter to a set of Pokemon. There were concepts that focused on the different roles a base and Mega form could have, and a couple were even on the slate; I see no reason to try and tack those concepts onto this one, especially when this is the first Mega CAP. It's already a big milestone, so going the safe route isn't a problem at all when we're in uncharted territory. There will be plenty of time to take risks imo.
I dislike people's reasoning that if the base form isn't great, that we might as well make a CAP with Mega stats. We should remember that we are looking for a mediocre-at-best typing that has a specific utility in countering specific Pokemon. That already alters how people play the CAP, as looking for the free turn to Mega Evolve is definitely a matter of timing and opportunity cost; the effect against item altering moves, like Knock Off, and generally high BST are a bonus. Yeah, you'll have to use your brain for this concept, even if there are no mind games to play. Finally, the concept isn't "How to best make use of the turn you Mega Evolve", "What are the benefits of a base form being different from a Mega form", "Mega Mind Games" or similar: it's looking at an undervalued typing an playing its strengths and weaknesses by way of being a utility counter to a set of Pokemon. There were concepts that focused on the different roles a base and Mega form could have, and a couple were even on the slate; I see no reason to try and tack those concepts onto this one, especially when this is the first Mega CAP. It's already a big milestone, so going the safe route isn't a problem at all when we're in uncharted territory. There will be plenty of time to take risks imo.