A "transformation" will never be a gameplay mechanic with summoners (at least not a primary one) because then its not an act of "summoning" anything. I can see a "spiritual fusion" of sorts, IE. - they get summoned, spirit of player enters the summoned creature's body temporarily to assume control bla bla blah - but not a "transformation".
My personal preference would be to see summons as something extremely powerful but extremely hard to get. Something like "only one summon can be 'equipped'" to take out with you in the field or whatever. One of the things that disappointed me about FFXI, is everyone could be a summoner but everyone was balanced. Summoners in the FF series have always been very powerful compared to non summoners - at least until very high levels when summons might become less useful compared to say, "the best weapon in the game" or whatever. For example, in FFVII, summons were the key to winning most fights for most people. However, later on, you learn that you can actually do more damage, faster, with omni-slash and a couple hero drinks than you can with knights of the round or a Bahamut-W-Summoning. In general though - you play a summoner in FFXI and feel fragile and weak soloing until very-high-level, when suddenly you become 'good enough' - and even then its still only average in most situations.
I guess I want to see summons actually be in line, at least for the most part, with 'the peak' of powerful things in the game that anyone can participate in experiencing, instead of 'just another job'.
EDITED IN PS. - To see what FF is without summons, watch spirits within... ugh.
Edited, Oct 12th 2009 4:43pm by FUJILIVES
Posting from just above the generator.